Abstract: For pursuing an appropriate numerical method for free surface flow with large deformation, a vertical two dimensional corrected MPS numerical model for the simulation of the dam break flow was built based on the Moving Particle Semi-implicit (MPS) method. A new free surface detection method and a highly precise pressure gradient model were proposed to mitigate the issues of particle misrecognition and numerical energy dissipation in the original MPS method. Furthermore, the influence of the source term of pressure Poisson equation on the shocking pressure associated with the dam break flow was discussed by the comparison with the experimental results of Lobovský et al., and then a new form of source term was proposed. The calculated results show that the new free surface detection method and the highly precise pressure gradient model have remarkable effects on the decrease of particle misrecognition as well as numerical energy dissipation. Moreover, the comparisons of the numerical pressure calculation and the experimental result reveal the proposed source term of pressure Poisson equation is useful for the reduction of the numerical pressure fluctuation.