Abstract: Abstract: Hydro-mechanical compound transmission has the advantages of good hydraulic stepless speed regulation performance and high efficiency of mechanical transmission, which can achieve the optimal matching of transmission system load and power source, and has been widely used in agricultural vehicles, engineering machinery and military vehicles. Domestic and foreign scholars have studied the speed control method of pump-motor pure hydraulic transmission system, the application and basic characteristics of hydro-mechanical compound transmission system, but the research on the control method of variable speed input-constant speed output of hydro-mechanical compound transmission system is relatively little. In this paper, a double feedforward and fuzzy PID speed control method based on hydraulic subsystem, mechanical subsystem hydro-mechanical compound transmission system was proposed. The speed compound control method was used to solve the problems that the output speed of hydro-mechanical compound transmission system was poor at the variable speed input and was difficult to control. According to the requirements of the output speed stability control of hydro-mechanical compound transmission system, the displacement compensation amount Δγ of the variable motor was calculated by the sum of the feedforward compensation amounts of the mechanical subsystem and the hydraulic subsystem: the feedforward compensation amounts was converted by the planetary gear of the mechanical subsystem planetary speed coupling device to the variable speed compensation amount Δnm(S of the variable motor through the planet carrier, and the corresponding compensation amount of the swash plate angle Δγm(S, the feedforward compensation amounts of the hydraulic subsystem was the variable caused by the hydraulic subsystem quantitative pump speed disturbance. The variable motor speed compensation amount Δnm(p was corresponding to the displacement angle compensation amount Δγm(p. Under different speed compensation paths, the stability of the system output speed was adjusted by the displacement adjustment of the variable motor. The simulation of the dual feedforward and fuzzy PID speed control method was carried out by AMEsim and MATLAB software, and hydro-mechanical compound transmission system test bench was built. The initial PID parameters, kp = 1.0, ki =0.1 and kd =0, of the system PID were determined through the simulation and experiments, the system input speed was maintained at 1 000 r/min by adjusting the initial swing angle of the variable motor displacement at the initial input speed of the system of 500, 600 and 700 r/min. Applying step signals of 500→530, 600→630 and 700→730 r/min to initial input speed respectivly, and traditional PID speed control method and double feedforward and fuzzy PID speed control method were used to control the system output speed, the results showed that compared with the traditional PID speed control method, the system output speed stability and adjustment speed of the double feedforward and fuzzy PID speed control method were greatly improved, and the average amount was reduced by 39.8%, the stable adjustment time was shortened by an average of 35.53%, and the average steady-state error of the system output speed was between ±0.7%, and the variable motor displacement compensation and output speed fluctuation amplitude were greatly reduced, the maximum compensation amount was also slightly increased, and the sensitivity of the system control was obtained, meeting the requirements for output speed stability control. The double feedforward and fuzzy PID speed control method proposed in this paper had the effect of suppressing the output fluctuation caused by the input disturbance of hydro-mechanical compound transmission system, the control effect was obvious, and the applicability under the complicated working conditions was enhanced. Hydro-mechanical compound transmission system provides a reference for the design and application of agricultural machinery.