Hydraulic retarder braking torque is needed to be controlled. Retarder braking torque refers to oil volume in retarder chamber, thus the key of braking torque control is oil charging & discharging control. The hydraulic oil charging & discharging system should have fast accurate response and provide enough flow rate. To achieve the requirement, the system is often complicated, many parameters need repeated test to determine, which increases the R&D cost and extends the research cycle. This paper tries to find a time-efficient research method on hydraulic retarder oil charging & discharging control system. The complete system is divided into some parts and every part is analyzed respectively based on mechanical theory and hydraulic theory. To build the precise model of all parts, the AMESim software is used. According to theory analysis the appropriate AMESim hydraulic elements are selected to assemble part, and every part model is set in detail to reflect the characteristic accurately. Finally, the complete system model is built and is used in hydraulic retarder braking simulation, the simulation result shows this system has fast response and good oil charging & discharging control performance.