Abstract: Input and output characteristics of ZL50 loader were measured under three working conditions of no load, full load, and half load. In every case, the matching calculation of engine and torque-converter was carried out. Then the traction force calculation of ZL50 loader was finished. The shift schedule of three variable parameters coming from the throttle position, the speed and the pressure of oil pump was introduced and the shift rule of ZL50 loader was designed. The dynamic simulating model of loader power transmission system was established. Simulation results indicate that the auto shift schedule of three variable parameters accords with the practical working state of construction vehicle and can meet the dynamic requirements.