Abstract: This paper provided a simple structure and high efficiency multi-range hydrostatic mechanical transmission composed of a single common planetary gear train for farm tractors. The changing characteristics of speed and torque, the synchronized range shifting condition and the distribution ratio of hydraulic power were analyzed. The utilized instance presents that the continuously variable speed range is wide. The distribution ration of hydraulic power is less than 20% in the common work speed range of tractors. There are three high efficiency pure mechanical gears in every range of low, middle, and high speed corresponding to cultivating, plowing and transporting. It is useful to improve power and economic capabilities of vehicles.