Abstract: For the quasi-static load computation of spacecraft under random vibration environment, the part of high frequencies contributes little to the random vibration load and could be cut off in computations. However, the selection of the cut-off frequency has to be determined. For that purpose, the assemblies with different dynamic characteristics are designed, and the random vibration tests are carried out on the assemblies with different cut-off frequencies. Based on the test data, the variations of the strain of the assemblies in different excitation frequency bands are analyzed. It is shown that, for general assemblies, the cut-off computation frequency is supposed to be 1.5 times of the main frequency of the assembly. For the uneven acceleration power spectrum input, we have no direct computation method, therefore, the frequency band method is proposed to calculate the random vibration load, and the contributions from different frequencies are analyzed, to determine the influencing factors of the cut-off frequency. The computation example indicates that the frequency band method could be used to correctly calculate the random vibration load.