Sunshine duration is an important observation for ground-based meteorological observations. The World Meteorological Organization defines it as "the sum of the periods of direct solar irradiance at or above 120 W/m²". The instrument for measuring sunshine duration is the sunshine recorder, and nowadays, the mainstream sunshine recorder in the world is the photoelectric sunshine recorder. The primary calibration method is the outdoor calibration method. This method has significant shortcomings, such as being affected by different cloud conditions and sky scattered radiation value is not constant, which results in low calibration test accuracy and significant errors. The experimental results show that the threshold error range of outdoor calibration of the sunshine recorders is 9.3~10.3 W/m², and the measurement results are difficult to achieve repeatedly^[1]. Hence, the study of indoor calibration systems that are not subject to environmental conditions is imminent. According to the measurement principle of the photoelectric sunshine recorder, the test system consists of 3 modules: direct radiation simulation system, radiation environment simulation system, and radiation suppression system. This paper focuses on the working principle of the radiation suppression system. The integrating sphere model was chosen as the initial model for the radiation suppression system. Because the distribution of radiation out of the integrating sphere is relatively uniform, even if a considerable amount of radiation returns to the radiation environment simulation system, the impact on the calibration test can be offset from each other. In this section, the calculation method of the radiation flux out of the integrating sphere is studied. Then the configuration of the radiation suppression system is analyzed according to the formula. According to the law of conservation of radiance and Taylors Series, the estimation formula for the flux at the exit port of the integrating sphere can be obtained. Before deriving the output radiation flux calculation, the effect of the thickness of the integrating sphere port in the radiation transmission is analyzed. From the direction of transmission, the radiation emitted from the integrating sphere consists of three parts: the radiation directly emitted to the outside of the integrating sphere, the radiation reflected by the port to the outside, and the radiation back to the inside of the integrating sphere. On the basis of the analyzed the direction of transmission in the integrating sphere, the radiation flux calculation model is established. Then the radiative heat transfer coefficient is introduced to transform the problem of solving the incident radiation flux into an analysis of how the radiation is transmitted between the two surfaces. By substituting the radiative heat transfer coefficients between each surface into the calculation model, the equation for estimating the radiation flux in a plane at a certain distance from the exit of the integrating sphere is obtained. Based on the formula, under the condition that the radius of the exit port, the thickness of the port, and distance from the exit is known, the larger the radius of the integrating sphere, the smaller flux back into the radiation environment simulation system, which means the smaller the error brought to the calibration. Using Light Tools software to build models, the radius of the integrating sphere was taken 1 m, 2 m, 3 m. The results show that the integrating sphere can suppress the radiation, and the sphere's radius is proportional to the suppression effect. The experimental results show that the accuracy of direct radiation simulation of the improved system has improved by more than 80%, and the absolute error is less than 2 W/m²; the accuracy of scattered radiation simulation has improved by more than 95%, and the absolute error is less than 1 W/m². The error of the system consists of two parts: 1) Stray light of solar simulator. But this error is essentially a systematic error generated during the alignment process of the solar simulator and coupled with the fact that the specific parameters of the integrating sphere vary. It can not be calculated precisely at present and can only be determined through actual measurements. It is recommended that the measured value of stray light be used as the threshold correction value for the current system. 2) The radiation suppression device. It appears from the measured data that the scattered radiation measurements are uniform. The principle of the photoelectric sunshine recorder is the total radiation minus the scattered radiation. Therefore, in the case of a uniform distribution of simulated scattered radiation, even if some radiation returns, the effect on the calibration results is negligible.