Computational fluid dynamics (CFD) based on the finite volume method (FVM) is used to simulate the three-dimensional viscous flow in a torque converter. The simulation result is in good coincidence with the test value and is analyzed theoretically. First, rotor enthalpy is applied to evaluate the hydraulic performance of the torque converter. It is effective to locate and classify hydraulic losses according to the drop in rotor enthalpy. Then, the mechanisms of the jet-wake and secondary flow are analyzed. There is strong coupling between the jet-wake and the secondary flow. The jet-wake flow begins with low-speed flow at the suction-core corner, which is transported to the suction side of the pump blade by the secondary flow. Finally, a prediction model of mainstream vorticity charging with pitch-wise and span-wise components is deduced from the momentum equation. The prediction model reveals that there is a positive mainstream vorticity in the first half of the pump flow channel and a negative value in the second half, which is consistent with the CFD result.