According to the structural characteristics of the entrance optics of self-developed solar spectral irradiance meter, the factors that introduce cosine error were analyzed. The cosine correction methods of direct irradiance, diffuse irradiance and diffuse-to-global ratio were studied. Laboratory measurements of cosine characteristic of solar spectral irradiance meter and a field campaign in Dunhuang with a variety of instruments involved were carried out. The results show that the cosine error is related to the structure and the black anodized inner wall of the input aperture, and it ranges from 4.3%~9.1% at the incident angle of 60° at 440 nm, 500 nm, 670 nm and 870 nm. Atmospheric optical thickness of the four bands retrieved from the direct irradiance obtained by solar spectral irradiance meter is severely affected by the cosine error, and the deviation of the atmospheric optical thickness before and after cosine correction is 0.11~0.13 and smaller than 0.012, respectively, compared with the CE318 sun photometer inversion results. Four-band diffuse irradiance increases by 6.8%~10% after cosine correction by assuming an isotropy distribution of sky radiance. Based on the sky radiance distribution data, an accurate correction method for diffuse irradiance was proposed. The simulation shows that the diffuse irradiance after cosine correction is in good agreement with the theoretical result, which proves the feasibility of the method.