Abstract: An electric drive power transmission system box assembly is provided having stepped pin positioning and a mounting method of the electric drive power transmission system box assembly. The box assembly includes a housing shaft string assembly, stepped pins, a first housing, and a first bearing. The housing shaft string assembly includes a housing and a shaft string supported on the housing. Each stepped pin includes a matching section and a guide section, the diameter of the matching section is greater than the diameter of the guide section, a tail end portion of the matching section is fixed in a fixing pin hole of the housing, the first housing is mounted and fixed to the housing shaft string assembly under the guide of the guide section, and a rear end of the shaft string is supported on the first housing by means of the first bearing.