Hydraulic system for off-line test of main steam isolating valve of nuclear power plant and test method

The invention discloses a nuclear power plant main steam isolation valve off-line test hydraulic system and a test method, and relates to the technical field of main steam isolation valve debugging, the nuclear power plant main steam isolation valve off-line test hydraulic system comprises a hydraulic pump, a first oil way of the hydraulic pump is sequentially connected with a one-way valve, a first electromagnetic reversing valve and a second electromagnetic reversing valve; a second oil path of the hydraulic pump is connected with an overflow valve and an electromagnetic unloading valve, and the overflow valve and the electromagnetic unloading valve are connected back to the oil tank; the first electromagnetic directional valve is connected with a main steam isolating valve through an oil inlet pipe, and the main steam isolating valve is connected back to the oil tank through an oil return pipe and a large-flow valve group; the first electromagnetic directional valve is further sequentially connected with a