Hydrodynamic torque converters for passenger cars were designed with an increasingly narrower profile for the purpose of achieving smaller axial size that would cause the change of inner flow field and performances. Understanding the change rules under different flatness ratio could guide the design of flat torque converter. To investigate the change rules, four different flatness ratio torque converters were designed by means of bran-new flat torus design method and blade design method based on quadratic function distribution of momentum. Flatness ratio was also redefined, and then the change of flatness ratio can completely reflect the change of whole torus. The internal flow field of the flat torque converters was calculated by CFD software. The character of the flow field in pump, turbine and stator was analyzed in detail. Through the analysis, several valuable conclusions were educed. Based on the numerical solution, the performances of the flat torque converter were predicted. By comparing with the performances of different flatness ratio torque converters designed by same method, the influence of the flatness ratio of torque converter on hydrodynamic performance was found out.