The spreading resistance is a very important parameter in the applications of heat sink. The design of electronic devices will fail without considering the influence of the spreading resistance. In this paper, a simple thermal model was simulated by Computational Fluid Dynamics software. Some factors, which have great influence on the spreading resistance, have been analyzed. The spreading resistance decreases significantly with the increasing of the area ratio between the heat source and the base-plate. While the ratio being 1, the spreading resistance reaches the mix value. The greater the thermal conductivity of heat sink, the lower the spreading resistance. With the increasing of the thickness of base-plate, the spreading resistance reduces. However, if the thickness exceeds the critical value, the spreading resistance will increase. And the spreading resistance reaches the mix value while the centers of heat source and the base-plate are overlapped.