To improve the real-time detecting precision of the wear depth for spherical plain bearing testers, a self-made spherical plain bearing tester was selected as studied objective, and its wear depth detecting system was analyzed and molded. The testing principle of the wear depth of the spherical plain bearing was introduced and the error factors affecting the wear-depth detecting precision were analyzed. Then, a comprehensive error model of the wear-depth detecting system for the spherical plain bearing was built by the multi-body system theory(MBS). In addition, the loading deformation of the wear-depth detecting system caused by the varying loading was detected. Finally, according to the above experimental parameters, the loading deformation of the related parts of the comprehensive error model was calculated by Finite Element Method(FEM). By comparison, the maximum difference between the experimental results and the calculated values is 0.028 mm, and the minimum difference is 0.012 mm. On the whole, these values are closer, which verify the correctness of the comprehensive error model under the loading error experimental conditions.