The traditional robot calibration method can only identify the errors of geometric parameters under no-load conditions.When the robot is deformed under heavy load conditions，the positioning accuracy is significantly reduced. This paper proposes a model of joint deformation for robots under heavy load conditions，the robot joint stiffness matrix is identified by the laser tracker and the control law design is optimized. Based on the POE model and the differential error model，the structural parameters are calculated under no-load conditions，and the result is written back to the robot controller. Under the heavy load conditions，the robot stiffness matrix is identified based on the previous transformation. The algorithm has been verified on the products of many robot companies. The results show that the calibration method can improve the absolute positioning accuracy of large load robots under heavy load conditions