In order to improve measurement accuracy of solar radiation observation instrument, aiming at cosine error caused by characteristics of diffusion and Fresnel diffraction,a method of correcting cosine in case of different installation forms and different structures was researched. Considering the solar radiation characteristics and cosine corrector production factor, cosine error correction method was proposed in installation forms of cosine error exposed and cosine error exposed cooperating stop ring, while the method in two structures of cosine corrector and integral cavity was proposed. According to the requirements, cosine error was modulated and simulated by different installation methods by TracePro software. Using high collimation solar simulator, turntable and rotating arm, different directions sunlight were simulated, and the cosine error was measured. The results show that in the installation forms of cosine error exposed cooperating stop ring, the incident angle is less than ±80, the cosine error is better than 6%. The correction method can reduce cosine error effectively with the condition of large incident angle, and improve the accuracy of of solar radiation observation.