Dongfengxisha reservoir, the important water source project in Chongming Island, is taken as the study case. An oil spill risk indicator system containing risk source, risk receptor and risk response is established based on PSR model. Pressure layer selects 3 indicators of access probability, total leakage and oil types. State level selects 2 indicators of water supply days and water replenishment time. Response layer selects 2indicators of ability to respond to emergency plans and emergency response time. By applying AHP model, the weights of different indicators are determined and sudden oil spill risk levels are discriminated, to provide the reference for daily water supply on Chongming Island and risk prevention for the Dongfengxisha reservoir.