Water injection well casing pressure relief device and water injection well

The utility model discloses a water injection well casing pressure relief device and a water injection well, the water injection well casing pressure relief device comprises a three-phase separator, an oil tank and a sewage tank, the three-phase separator is provided with an inlet pipe, an exhaust pipe, an oil discharge pipe and a water discharge pipe, the inlet pipe is used for connecting a casing pipe of the water injection well; the oil tank is connected with the oil discharge pipe, and a stop valve is arranged between the oil tank and the three-phase separator; the sewage tank is connected with the drainage pipe, and a stop valve is arranged between the sewage tank and the three-phase separator. When the casing pressure relief device for the water injection well is used, an annulus mixed with crude oil, sewage and gas flows out of the casing pipe and enters the three-phase separator, the gas is separated and discharged from the exhaust pipe, due to different densities, the sewage and the crude oil are sep