The relation between distribution of belt friction force and belt slip velocity was simulated by FE model. It was found that when pulley thrust was gradually reducing, sliding slip between the pulley and all elements occurred before belt friction coefficient reached the maximum point. When sliding slip occurred, the friction force in the radial direction of pulley existed. As sliding progressed, the direction of friction force accorded with the tangential direction of pulley, and the belt friction coefficient reached the maximum point. This characteristics simulated by FE model corresponded well with measurement results, verifying the slip mechanism.