A method is proposed based on CFD to calculate axial force of hydrodynamic coupling. By using the numerical solutions of hydrodynamic coupling inner flow field, which is unsteady, incompressible and viscous two-phase flow, the axial force is calculated. The flow field in working chamber of variable speed hydrodynamic coupling is analyzed in different filling rates under breaking condition, traction condition and rated condition, and the variation low of axial force is obtained. Axial force is measured by experiments in variable speed of high-power hydrodynamic coupling. The comparison of the calculating results with the experimental ones shows that the mean error is 8%, and the calculation method based on CFD is feasible.