A device for mounting and fastening a circular nut on an output shaft of a planetary gear reducer and measuring the moment of resistance of the output shaft mainly comprises a wrench frame column, a wrench holder, a circular nut torque wrench, a hydraulic wrench, a turnover device cylinder body, a turnover device fixture, a reducer torque transducer, a bevel gear, a pull rod fixing frame, a pull rod and the like. Through the adoption of the device, the circular nut can be mounted on the output shaft of the planetary gear reducer without manual striking any longer, the mounting process is enabled to be simpler and faster, and the damage to the nut during striking is reduced, so that the production process is improved. The device also has the advantages of high reliability, convenience in use and improvement of production efficiency.