The wheels will idle when the relative slipping speed between the wheel and rail exceeds the reference slipping speed. In order to avoid this phenomenon, the simplified model of wheel-rail traction torque transmission was established. And the adhesion coefficient and vehicle velocity are got through the disturbance observer. Then the recursive least squares method was used to forecast the slope of the adhesion-slip curve. Sliding variable structure controller was used to control the error of wheel velocity and reference velocity. From the results of simulation, this method can be effective to maintain the adhesion coefficient around the maximum. And the slipping speed approached the reference value, so the damage for wheel and rail was effectively prevented which achieved the desired effect.