High-pressure physics has advanced significantly since its inception, initially being used to study the structure of materials within the Earth. Today, it has applications in various scientific fields, such as chemistry, physics, biology, materials science, and pharmacy. Through the development of advanced instruments and software, high pressure research has become more precise, comprehensive, and complex.Compared to the momentary pressure changes brought by dynamic high pressure shock, static high pressure loading has the advantages of safety, cleanliness, and low response time requirements for detection equipment. In the late 1950 s, high pressure science entered the era of Diamond Anvil Cells (DACs). In the following decades, DAC technology has been continuously improved, including increasing the chamfer of the opposing anvils to further increase pressure, improving the metal gasket materials, and introducing pressure transmission media. These improvements have increased pressure while solving problems such as pressure gradients and sample leakage. The continuous improvement and development of these technologies have not only expanded the upper limit of pressure that experimental techniques can achieve but also greatly promoted the development of high-pressure science. Diamond anvil cells, due to their unique structure, can generate high pressure on a small area and are widely used in studying material properties under high pressure. In-situ optical radiation measurement techniques in diamond anvil cells include high-pressure Raman spectroscopy, high-pressure synchrotron X-ray radiation, photoluminescence spectroscopy, ultraviolet-visible absorption spectroscopy, and infrared spectroscopy. However, most existing optical detection methods for diamond anvils require expensive large-scale detection equipment, such as synchrotron radiation sources. Furthermore, these methods are often focused on the absorption spectrum or photoluminescence properties of the materials themselves without directly measuring the material's optical constants.This study aims to obtain refractive index data for materials inside the diamond anvil cell under different pressures through in-situ Small-Angle Ellipsometry (SAE). Ellipsometry is a non-contact, non-destructive optical detection method that modulates the polarization state of the light beam when reflected or transmitted on the sample surface. By detecting the polarization state changes of the incident light beam before and after reflecting from the sample, the proposed SAE obtains information such as the optical constants of the thin film to be measured.To achieve this goal, the study designs an in-situ small-angle ellipsometry system to satisfy the special measurement conditions of limited incident angle, long working distance, and miniature sample size under high pressure. The system achieves small-angle ellipsometry in the 450~700 nm spectral range. It monitors the changes in the Stokes vector of the incident light beam before and after reflection from the sample inside the diamond anvil cell. It obtains the reflectance ellipsometric parameters ψ and Δ for isotropic samples under pressure loading.However, due to the actual multiple-layer film transmission inside the diamond anvil cell, the iterative analysis formula for complex refractive index calculation is too complicated the transfer matrix method is employed for calculation. The polarization state of the light beam is described by the Jones vector. After reflection, transmission, and interlayer transmission, the polarization state of the light beam changes and can be described by a 2×2 Jones matrix. Theoretical ellipsometric parameters are then extracted based on the matrix calculation results. The study uses numerical simulation software to set a fitting evaluation function and obtains the real part of the refractive index information for the sample under test. The study measures four sets of information for real-time loaded diamonds under pressure and calibrates the loading pressure value using the diamond Raman peak.Finally, the real part of the refractive index information for single crystal silicon under a pressure range of 2~9 GPa is obtained. The study also performs in-situ Raman detection on the same sample. The experimental results show that the refractive index of single crystal silicon increases with increasing pressure in the pressure range of 2.56 GPa to 8.81 GPa, which is consistent with the redshift trend observed in the Raman spectra.The development of this small-angle in-situ ellipsometry system provides a useful supplement for the optical constants and in-situ measurement of materials under high pressure, which is critical for exploring the optical properties of other thin films on silicon substrates under high pressure and for understanding the interplay between pressure and optical properties. Moreover, the small-angle in-situ ellipsometry system designed in this study can be used for in-situ measurement of the optical properties of other materials under high pressure, expanding the scope of high-pressure research and promoting the development of related fields.In summary, high-pressure physics plays a crucial role in various fields, and the study of materials under high pressure provides important insights into their properties and behavior. The use of diamond anvil cells and in-situ optical radiation measurement techniques have significantly advanced high-pressure research, but there is still a need for new, more efficient, and cost-effective methods. The small-angle in-situ ellipsometry system developed in this study provides a promising new approach for in-situ measurements of material properties under high pressure. By obtaining refractive index data for single crystal silicon inside the diamond anvil cell, this system can provide a new perspective and help researchers gain a better understanding of how pressure affects the optical properties of materials, which can have significant implications for a wide range of applications.