According to the difference of the tractor operating mode，the hydro-mechanical automatic transmission can automatically complete the change of transmission ratio and achieve the match of the speed and torque of the engine under different operating modes. The forward section of the designed hydraulic machinery adopted four sections of geometric proportion. Each section transmission common ratio was 1.88. Pure hydraulic section of the designed hydraulic machinery was used for the start of the tractor. Based on the continuity of the transformation of different segments of speed ratio, the gear ratio and power match of variable displacement pump and quantitative motor of each drive pair was determined. Also, the load characteristics of the tractor under various operation modes were combined. The speed ratio match of the optimal economic point of the engine of four sections of the hydraulic machinery was determined. Then， the continuity of the segments of transmission between different sections of hydraulic machinery and continuity output response of smooth acceleration of the tractor were verified. 