When the vehicle works in a long downhill operation, in order to keep the heavy loading vehicles continue steady brake, it is usually necessary to install the hydraulic retarder device to ensure that the vehicle is able to brake for a long time without overheating. The distribution of internal flow field of the retarder directly affects the performance of the mechanism and the available brake torque. Based on its internal structure and work characteristics, CFD of computational fluid mechanics was used to construct its model and to analyze the internal and external full flow field, and the braking torque at different working conditions was calculated. On the basis of the model analysis, the test bed was set up, and the reliability and accuracy of the simulation moment analysis were verified through test analysis. The results show that the overall distribution of internal flow field is reasonable; the operation speed and filling rate are the important factors affecting the braking torque of the mechanism. The test results verify the accuracy and reliability of model simulation and provide reference for similar research.