Laser shock forming (LSF) is a novel technique realized by applying a compressive shock wave generated by laser shock on the surface of the metal. It is a mechanical, not a thermal process, and has a significant characterization of ultra-pressure and super plastic forming etc. In this paper, a physical model and the estimated formula of shockwave pressure of laser shock forming were found according to the detonation wave and explosive gas dynamical theory. On the base of experimental results and theory analysis, a mathematical model of the sheet deformation under one laser shot loading was established, the relationship between shockwave pressure and deformation of sheet metal was discussed, and the effects of factors such as laser parameters, boundary condition on the deformation of sheet metal were analyzed. These model and formula are very useful and can be used to predict the deformation of sheet metal loaded by laser shock, and provide a theory foundation to choose laser processing parameters reasonably and program the controlled software of laser shock forming.