Abstract: A free surface traced method was put forward for moving particle semiimplicit (MPS) method. In simulation of MPS, the free surface is traced by the number density parameter between the explicit step and implicit step, and the number density parameter is changed because of the first explicit step, some nonfree surface particles are treated as free surface due to the small number density parameter which is caused by explicit step. The new free surface traced method is detected by neighbors, the zone of a particle is divided into eight parts, and the particle is treated as free surface when no neighbor particle in any two neighboring parts. Taking into account that the number density parameter has a higher efficiency for free surface, the neighbor detected method is mixed with the number density parameter, and the neighbor detected method is only used in the range of number density when mistreatments are most likely to happen. The result shows that the neighbor detected method is efficient for free surface, and mixed use only has a little effect on the efficient of the simulation.