The invention discloses a plunger piston hydraulic pump motor rotor dual alloy structure diffusion welding method, which comprises the following steps that 1, the dimension of a workpiece to be welded is subjected to pretreatment; 2, the gasoline soaking ultrasonic processing, the blowing drying, the acid washing ultrasonic wave processing, the water flushing, the absolute ethyl alcohol soaking ultrasonic wave processing and the blowing drying are sequentially carried out; 3, a nickel foil is adopted to be used as a welding transition layer material, and a rotor steel substrate, a copper rod and a rotor end surface copper plate are assembled; 4, through the axial pressurization, an assembled rotor welding assembly is placed into a welding hearth to be subjected to diffusion welding. The method provided by the invention has the advantages that through the conventional diffusion welding on a rotor, a welding joint meeting the use requirements can also be obtained, and in addition, the problems caused by welding deformation can be avoided.