The hydraulic retarder is a crucial auxiliary braking component on heavy-duty trucks. However, the hydraulic retarder has poor braking performance over long distances and heavy loads. The internal flow field distribution directly affects the hydraulic retarder's working performance and braking torque. In this paper, we aim to analyze the influence of the structural parameters of the stator and rotor on the braking torque. The entire flow path of the hydraulic retarder was selected as the computational model. The reliability of the simulation model was verified through grid independence validation, and the accuracy of the simulated torque was validated through institutional test bench experiments. We analyze the changes of pressure field, velocity field, and turbulent kinetic energy, then explore the relationship between the internal flow field and output torque. The results indicate that the angle of inclination contributes to the torque to a certain extent. The maximum braking torque occurs at 40°. The severe flow separation occurs at 45°. Compared to the initial model, the braking torque loss at 45° is 13.7%. The turbulent kinetic energy of the 45 mm inner diameter was increased by 55.92% compared to the initial model. However, the 45 mm inner diameter results in unstable flow and a loss of 14.93% in braking torque.