In order to avoid the difficulty of testing the reflectance of spherical mirror,two methods to establish the SURSI irradiance calibration are introduced. One is the method by using indirect parallel light and the other method is by using the direct diverging light. Detail descriptions of calibration processes of the two methods and a series of comparison and analysis of the calibration results are given. The relative deviation of calibration results between two methods is 1.0%. The absolute uncertainty budget of the 250-400 nm spectral irradiance calibration using the parallel light method is ±2.8%,and the relative uncertainty budget is ±2.0%. The budgets using the diverging light method are ±2.4% and ±1.4%,respectively. The relative deviation value is less than that of any one of the two relative calibration uncertainties. The analysis result shows that the direct diverging light method to calibrate the space ultraviolet remote sensing instrument (SURSI) is more effective to reduce the radiometric calibration uncertainty compared with the indirect parallel light method.