Accurate spectral irradiance measurement is very important in the field of Earth observation. In optical radiation measurement, a cosine diffuser is often used to improve the accuracy of irradiance measurement results at different azimuths. The cosine response characteristic is the key factor of the spectral irradiance sensor in the field measurement. This paper studied the cosine response characteristics of the commonly used spectral irradiance instrument for ground object observation, and a spectral irradiance experimental measurement system was established. The variation rules of the instrument'sspectral irradiance measurement results at different angles were obtained through experimental methods, and the cosine curve was fitted by MATLAB software. The influence of the traditional angle response normalization method and the offsetting angle response normalization method on the instrument cosine error was analyzed. Compared with the traditional normalization method, the maximum cosine error of the spectral irradiance instrument was reduced from 11.2% to 7.7% by using the offsetting normalization method, and it was almost independent of the wave length changes. This is mainly due to the limitation of the optical path structure inside the instrument and the diffused material. When the incidence angle θ changes, the spectral irradiance angle response deviates from the cosine function. Therefore, based on the cosine response characteristics of the instrument and the experimental results of cosine error, a method for correcting the cosine error of the spectral irradiance was proposed. By extrapolating the single wavelength fitting function to all bands for correction processing, the deviation between the correction value of the instrument's spectral irradiance and the standard cosine response curve is less than 1.7%, and the cosine error is reduced from 8% to less than 2% in the range of -55~55°. The correction method greatly reduces the influence of cosine error on the instrument's measurement results, improves the instrument's measurement accuracy under different azimuths, and meets the high precision application requirements of field measurement, such asearth observation and ocean remote sensing.