The invention relates to a pressure relief and water drainage device of a manifold of a diesel oil water pump unit. The problems that in an existing diesel oil water pump unit, after well site work iscompleted, too much water is left in an environment-friendly impounding reservoir and a water conveying pipeline, and the water cannot be drained are mainly solved. The pressure relief and water drainage device is characterized in that two water pumps (2) are connected with a first-stage impounding reservoir (9) through water inlet pipelines (1); water outlet pipelines of the two water pumps (2)are connected together through an inlet and outlet pipeline (4); the inlet and outlet pipeline (4) is connected with an environment-friendly impounding reservoir (10) through two water outlet pipes (11); valves are connected to the water outlet pipes (11); check valves (5) are connected to the water outlet pipelines of the two diesel oil water pumps (2) correspondingly; the inlet and outlet pipeline (4) is connected with the first-stage impounding reservoir (9) through a discharging pipe (3); and a valve (6) is connected to the discharging pipe. According to the pressure relief and water drainage device of the manifold of the diesel oil water pump unit, a great amount of water stored in the water outlet pipelines and the environment-friendly impounding reservoir can be smoothly drained out, environment pollution can be reduced, and safe production can be guaranteed.