Abstract: A mathematical model describing the interaction between immersed rigid body and fluid was established and was solved using the projection step method similar to the fractional step method for solving the incompressible Navier-Stokes equation. The volume ratio of a fluid grid was introduced to consider the effect of immersed rigid body,which was calculated by Romberg algorithm effectively.According to the numerical method,the corresponding numerical calculation program was written by C + + language. The reliability of solving the governing equation of projected immersed boundary method(IBM) was validated by the basic numerical example of flow over a circular cylinder. Finally,the distributions of the velocity field and the pressure field as well as the vortex structures were analyzed in different Reynolds numbers.