The invention provides a hydrostatic testing machine for engine assembly detection and an application method. The testing machine comprises a water tank, wherein the water tank is connected with a liquid supplementing circulation circuit, an exhaust circuit, a supplementary step-down circuit, a cooling liquid collection circuit, and a hydraulic pressure test circuit; the water tank is sequentiallyconnected with a first heat pump, a first water filter, a temperature transmitter, a first pressure transmitter, a first pressure regulating valve and an air cooling radiator to form the liquid supplementing circulation circuit; and a second water filter is sequentially connected with a first pressure gauge, a second heat pump, a heat exchanger, a temperature sensor, a third electromagnetic valve, a second temperature transmitter, a first manual ball valve, an engine/engine component, a second manual ball valve, a third temperature transmitter and a fourth electromagnetic valve to form the hydraulic pressure test circuit. The invention provides a set of reliable detection means for the quality control of the sealing performance of engine products, so that any cooling waterway sealing performance detection including the engine and the engine component can be realized, and the detection structure is high in accuracy and short in test time.