Abstract: Once an accident occurs in a large oil tankfarm, leaked oil and all kinds of sewage generated during accident treatment may induce pollutions to the surrounding environment. In order to avoid such situation, and accommodate old tankfarms to new standards, the water prevention and control system in PetroChina Dalian Petrochemical Oil Storage and Transportation Company is investigated. Capacity of key prevention and control facilities against emergencies (e.g. useful volume of emergency pond) is calculated, and the deficiencies of the system in case of tankfarm accident are identified. Then, corresponding improvement measures are presented to ensure that all sewage and materials in case of accident are all under control, so as to reduce the risk probability and maximize the economic benefit.