Unloading supporting system is important for semirigid structure, such as suspen-dome, during construction. Thus a failure criterion is presented for standard suspen-domes under unloading impact load. Using the criterion, analysis on the effect of unloading rate on the structure dynamic stability is developed adopting the Ansys software. The results that using direct unloading will be unsafe at some extent, improper unloading can cause structure failure are drawn. And further the structure failure mode and corresponding early warning system are obtained for unloading impact. Research for unloading failure mode and failure mechanism of a project case is presented, which verified the method obtained in the paper advisable and effective.