In order to solve the problem of fluid-structure interaction, this paper presents a new numerical method named as particle on grid method (POGM) for incompressible fluid. Based on the physical mechanism of the fluid motion, the POGM has a clearly physical meaning by using representative physical values at points to describe the fluid’s overall flow. It has been proved that this method is consistent with Navier-Stockes’ equation essentially, and some numerical problems have been discussed. Coutte flow and Poiseuille flow are used in the numerical examples. The results of the POGM are well agreed with the analytical solutions of incompressible flows of the examples.