When worked in field traction operation, the productivity index of the tractor unit is mainly reflected by the traction performance. Through analyzing the influence factors of the maximum productivity and the traction characteristics of tractor when equipped with automatic power-shift transmission, it is determined that the improvement of traction power of tractor is the fundamental measure to obtain the maximum productivity. Based on the principle, the dynamic gearshift rule and its control strategy are presented, which takes the throttle opening and engine speed as control parameters. Taking YTO LA2004 wheeled tractor as investigative subject, the simulation analysis of heavy load working condition such as ploughing was carried out. The results show that the maximum productivity shift rule can be used to shift the transmission gear with the change of traction load, which ensure that the engine works at speed governing line near the maximum power point, it can improve the traction power and traction efficiency of tractor. And when the load is small fluctuations, it doesn’t affect the change of gear, which can avoid the repeat gearshift phenomenon. The research provided a theoretical basis for the development of automatic transmission control system of tractor.

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