In order to avoid the influnce of accident water on sewage treatment system in a chemical plant,it is necessary to set up accident pool to store the sewage discharged in the event accident. The determination of the volume of the accident pool and the design scheme of 9.6×105 t/a of coking engineering accident pool in Shanxi Lucheng Yajin coking corporation Ltd. were introduced. The determination of the volume of the accident pool should be combined with the three-level protection system of the project ( pollution sources, processes and final emissions ), so as to control the sewage in the plant under the accident condition to ensure the safety of the environment.