A dynamic simulation of fiber reinforced composite mold filling process in a complex cavity is presented based on the gas-solid-liquid three-phase model for mold filling and the finite volume method on non-staggered grids. The interface evolution and the information about physical quantities, such as velocity, stresses, pressure, etc, are given. The motions of fibers, including transformation and orientation, are obtained as well. Numerical results show that fiber orientation in the horizontal mid-plane is related to the shape of the cavity, which is quite different from the case of the skin-core-skin structure orientation along cavity thickness. In the inlet region, fibers encircle the cavity inlet, and fiber orientation is vertical to the incoming flow direction along the horizontal and the vertical regions of the cavity, while around the corners of the cavity, fibers point to the corners of the cavity.