Abstract: The paper has analysed the influence of the structure of original main clutch in tester and its performance on testing accuracy, put forward an improvement design for main clutch and designed a controllable automatic device for force increment substituting for mannual converting main shaft, originally used for testing static friction factors. The properties of the tester improved approach to those of SAE No. 2 friction tester for clutch. Furthermore, the influence of slip linear speed on static friction factor has been discussed.