Abstract: A constant torque control of the air-fluid control system was studied for the hydraulic retarder. According to the relationship between control pressure and oil volume rate got from the hydraulic retarder experimental data, a model of the air-fluid control system, namely the oil volume control system model, was established for the hydraulic retarder based on AMEsim software. Then, a PID-based constant torque controller was also designed. Finally, combining the software AMEsim with Matlab/Simulink, a co-simulation was carried out to verify the constant torque control for the air-fluid control system of the hydraulic retarder. The simulation results show that, the retarder torque can remain constant when the target control torque of retarders is set less than the maximum retarder torque, and the torque controller can improve the control accuracy and real-time performance.