We present an easy approach to obtain the dynamic flow field with a moving body inside. Instead of popular interface boundaries, the interior boundaries were applied as moving parts, which made separated computation volumes a default-continuum meanwhile. With the combination use of UDF and dynamic mesh model, the numerical technique was tested and validated by simulating flow through moving screen, a kind of porous material. The pressure and velocity contours obtained clearly showed the changing behaviors of the flow while passing through a moving porous material, and clarified the influence posed by the movement of its frame. The method is used for the simulation of the flow with a moving body inside.