Pressure pulses generated by cavitation collapse when vehicles launching through the free surface will form the key load which decides the strength of structures. Consequently, it is very significant to investigate the mechanism of cavitation evolution and collapse. Firstly, a typical vertical launching process was simulated, and time sequences of pressure distribution gained were verified by experimental results. Through the analysis about flow fields, mechanism for the occurrence and evolution of cavitation collapse was investigated. Furthermore, a physical model of collapse pressure was established, by which influence of important factors such as cavity thickness, thickness of water layer, sound speed were studied. Finally, relative issues about the similarity law of scaled tests and load-reduction measurements were discussed.