As an excitation spectrum, Raman spectroscopy uses laser as an excitation source to excite Raman signals of all gas molecules. Due to the low molecular density, high transmittance of light and low Raman Scattering Cross Section, the utilization efficiency of the eycitation light energy is low, and the Raman signal scatters to space around focus, only fraction of signal can be collected by collecting system. As a result, the detection limit is poor and cannot be widely applied to the detection of gas. In this paper, a Raman right angle reflection cavity was proposed to improve the detection limit of Raman detection of transparent samples such as gases. The Raman right angle reflection cavity used a right angle mirror to reflect incident light back to the original direction but the optical path had a spatial offset. Two parallel to each other, oppositely placed right-angle mirrors were used, and the laser with a beam diameter of 0.7 mm had a working diameter of 25.4 mm. The exciting laser was reflected back and forth 10 times in the cavity, and two lenses were placed in opposite direction around focus which were used to focus the excitation light to the same focus, thereby improving the use efficiency of the exciting laser energy. The Raman scattering signal transmitted along with the direction of incident laser was reflected back by the right-angle mirror to the right about, after being focused by the lens to the focus, with the Raman scattering signal scattered to the laser incident direction all passes through the long-pass filter and collected by Raman spectrometer, thereby improving the collection efficiency of Raman scattering signals. The experiment was carried out with air as the test object. The Raman spectrum of clear carbon dioxide and the fine Raman spectrum of nitrogen and oxygen were obtained within 300 s and the intensity ratio was analyzed, including 2 332 cm-1 of nitrogen and 1 557 cm-1 of oxygen. The peak height ratio of the 1 388 cm-1 Raman peak of carbon dioxide was 785∶257∶1. The Raman right angle reflection cavity added two right-angle mirrors and one focusing mirror compare to the conventional Raman scattering excitation collecting system, and had the characteristics of small volume, simple structure and easy adjustment. The signal intensity distribution of Raman scattering to the surrounding space was related to the incident direction of the incident light, and the maximum of Raman signal accorded with the direction of the incident light and the reverse direction. The Raman right angle reflection cavity was designed to match the Raman signal intensity distribution, and along with the advantages of optical depth of field was utilized to maximize the Raman scattering signal collection efficiency. The Raman right-angle mirror cavity can extend the application of Raman spectroscopy in gas detection, such as in-situ monitoring of gas phase chemical reactions, engine combustion processes and emissions detection, and unknown pollutant gas analysis.