The parametric design and optimum design method of variable speed hydrodynamic coupling based on UG and CFD was proposed. The parametric model and optimum design flow of variable speed hydrodynamic coupling were established. The original characteristic curve based on flow-field numerical solution matched with the experiment result well, so CFD could be used for the complex gas-liquid two-phase flow of hydrodynamic coupling. Numerical simulation and performance prediction were conducted respectively to three different parametric schemes under brake condition, traction condition and rated condition, and compared with the base-type coupling. The second scheme had a more reasonable flow distribution, its pressure and velocity difference was smaller, pump torque coefficient was larger, and was the reasonable optimization scheme of base-type coupling.