The opto-mechanical  system design of self-developed irradiance spectroradiometer  in  reflective  solar  bands  is  introduced.  The  instrument  has  a  spectral  range  of 400–2 500 nm, including three spectrum 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. Compared 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 au-tomatic calibration of satellite remote sensor.