Abstract: To avoid the traditional body-fitted numerical methods and extend the application of immersed boundary method to complex flow, a new numerical method for simulating complex flow based on the implicit direct force immersed boundary method is presented. A mathematical model described the interaction between an immersed rigid body and fluid is established and whose governing equation is solved using the projection step method similar to the fractional step method for solving the incompressible Navier-Stokes equation. The moment source is not pre-calculated, but determined implicitly in such a way that velocity at the immersed boundary interpolated from the corrected velocity field accurately satisfies the no-slip boundary condition. Also, the second velocity near the solid wall updated is implemented using a δ smooth function. The QUICK upwind scheme and the second central scheme are applied to solve convection and diffusion terms respectively. The second explicit Adams-Bashforth method is used to the time discretization. The present immersed boundary method(IBM) is validated by the basic numerical example of flowing over one cylinder at Reynolds number 25,40 and 300.