The determination of vibration tests specifications is a prerequisite for rightly examination of the capability of spacecraft structure.The rigid boundary condition in the experiments is sometimes significantly different from that of the real mounting structure, resulting in the discrepancy of transfer function. Taking the envelope of interface acceleration as the specification would cause over-testing or under-testing condition. Load mapping method (LMM), which aims at the response equivalence, takes the real response and experiment boundary condition as the input and uses optimization algorithm to calculate the mapping load based on FEM model. It is found that LMM can solve the under-testing caused by boundary condition difference from the analysis and test results of a representative structure that has a flexible boundary condition. The over-testing at resonance frequencies of the payload can be reduced by notching method or Force Limit Method.