The opto-mechanical system design of self-developed irradiance spectroradiometer in reflective solar bands is introduced. The instrument has a spectral range of 400-2500 nm, including three spectral modules, and it can achieve automatic long-term measurement of total solar irradiance, diffuse sky irradiance, direct solar irradiance and the ratio of diffuse irradiance to full irradiance. In order to verify the rationality of the opto-mechanical design, we analyze the field experimental data. Comparing with the ratio data of the traditional manual measurement, it is found that the ratio deviation of the two methods is less than 2%. The instrument has the same irradiance measurement accuracy comparable to the traditional measurement method, and has certain advantages in the automatic calibration of satellite remote sensor.