The hydraulic flat torque converter is designed for the purpose of study on how the flatness ratio effect on the hydraulic performance. The flat torus new design method based on the ellipse is used. Four kinds of different flatness ratio hydraulic torque converter are designed. The internal flow field of the torque converter is simulated by CFD,and the distribution of the velocity and pressure of internal flow field is analyzed,and compared with the standard hydraulic torque converter,the influence of the flatness ratio of hydraulic torque converter on hydrodynamic performance is found out. With the flatness ratio decreased,the hydraulic performance of the hydraulic torque converter is getting downtrend.