On the basis of radiation theory,the result expressions of spectral irradiance responsivity calibration of space ultraviolet remote sensing instruments are deduced by diverging light illumination method and parallel light illumination method. The factors which impact the calibration results of two methods are analyzed. The calibration method error of diverging light illumination method is calculated through the relevant testing. The calculation results show that the calibration method error of diverging light illumination method is 0.5～1.2% in the assumed condition that parallel light irradiance is uniform and the diverging light source is the Lambert source. The calculation result has some guidable significance for carrying on spectral irradiance responsivity calibration of space ultraviolet remote sensing instrument by diverging light illuminating method.