Abstract: In order to deal with the possible water pollution emergency such as ship oil spill in downstream of Xiangjiaba Dam, Yibin river section was taken as a study area, based on the measured topographic and hydrological data, a two-dimensional model of oil spill diffusion was established to simulate the drift path and diffusion area of the oil film after a 20 t oil spill accident occurred on a ship 1 km downstream the dam. Results showed that: (1) During the navigation period, when the outflow from Xiangjiaba Reservoir increased, the time that the oil spill reached different area and pass through different locations was significantly shortened. The time that the oil spill reached to the Pu’an water plant was between 1.58 and 3.05 h, the time that passed through Pu′an water plant was between 0.28 and 0.78 h, the time that reached the Yibin hydrological station was between 3.45 and 8.62 h, and the time that passed through the Yibin hydrological station was between 0.33 and 1.62 h. (2) The diffusion area of the oil spill decreased and the thickness increased with the increase of outflow from reservoir in the process of oil film drift. The oil spill diffusion area that arrived at Pu′an water plant was between 0.141 and 0.224 km², and the average thickness was between 0.351 and 0.491 mm, the oil spill diffusion area that arrived at Yibin hydrological station was between 0.229 and 0.423 km², and the average thickness was between 0.205 and 0.328 mm. (3) The water surface area was small in the dry season, in which the oil film was easy to adhere to the shoals and wetlands on both sides of the curved river during the drift to the downstream. The total area of the oil film adhered to both sides of the river was 0.137 km² when the outflow from reservoir was 1 650 m³/s. As the outflow increased, the area of oil film adhered to both sides of the river became smaller. The research results could provide a scientific basis for the optimization of the emergency rescue plan for the water pollution emergency and oil spill accident in the Yibin river section.