The modified SMAC method is constructed based on CFD. It is also discussed and Applied to simulate two dimensional viscous unsteady incompressible flows. The SMAC methodFor complex flows with non-staggered grids arrangement, which is on the basis of the unsteady N-S equations, is constructed by introducing a vorticity preserving auxiliary potenial functiona and Momentum interpolation scheme. For pressure calculation, some modification is proposed.Pressure compatiblity is ensured with non-staggered grids by special of pressure PoissonEquation and boundary conditions. The two classic examples of unsteady flow, namely the flowOuer the square cylinder and the circular cylinder, which both have periodic result, are simulated.