The advantages of small volume and convenient installation,the hydraulic retarders are used more and more widely in the vehicle. Aiming at the integrated hydraulic retarder of the AT500 automatic transmission, its braking force control valve was optimized .The internal flow field of control valve was simulated by using fluid simulation software Flow Simulation . By changing the transition structure of spool shoulder, the hydrodynamic force generatd when control valve spool moves was reduced significantly.The spool could be stably stopped at any transition position under pilot control pressure, which provided the conditions for precise control of braking force.