When the tractor is under heavy-duty operation, the engine is operating at full throttle and in speed regulation the torque of the engine will change during the shifting process, causing sudden change of torque when clutch engages. Aiming at the solution of the problem of shifting gears under complex operation, the oil pressure control method of clutch synchronnization phase is proposed. The tractor's operating characteristics and power shift quality requirements are fully taken into account and the changes of vehicle speed, impact degree and sliding friction work are used as evaluation indicators. The genetic algorithm is used to analyze and design the power shift process of the tractor. The results show that all indicators of gear shifting process come up to the standard and the control method is correct and feasible.