Using a 3-D estuarine coastal and ocean numerical model with an oil film drift and diffusion module, the area, thickness and trajectory of the oil film was studied after an oil spill accident happened at the 300 thousand ton crude oil wharf in the Bohai Sea. The results show that in the case of no wind the oil film moves southwestward along with the tidal current, its area increases and thickness decreases, and the port is not polluted during ebb tide; a part of the oil film is blocked by the northern jetty, and stays in the port during flood tide. The oil film trajectory oscillates back and forth with the ebb and flood current, and its thickness decreases and area increases during the drift process with oscillation due to the diffusion. The change of wind speed and direction has a significantly influence on the oil film trajectory and pollution extent in the port.