To study the impacting mechanism of variable polarity frequency (VPF) on the weld pool stability in variable polarity plasma arc welding (VPPAW), the thermal-mechanical coupling process of the keyhole weld pool was analyzed by a variable-polarity finite element model. The model was developed based on the computational fluid dynamics (CFD), which can realize the periodic variation of thermal and mechanical effects of variable polarity arc on the weld pool. Moreover, in order to more accurately express the heat and force distribution on the keyhole boundary along the keyhole depth, the secondary compression effect of the keyhole on heat flux and arc pressure was taken into account. The thermal-mechanical “oscillations intensity” on the keyhole boundary and the force balance of the molten bridge were compared at different VPFs. The results show that the “oscillation intensity” of the arc pressure, heat flux and the flow velocity on the keyhole boundary decreases with increasing the VPFs during the keyhole formation. In addition, the variation of the temperature field in the weld pool caused by different thermal-mechanical oscillations results in the change of surface tension and the force balance condition of the molten bridge is changed, which affects the keyhole weld pool stability. When the VPFs are more than 33 and less than 83 and the duty radio of current in EP phase is 1/5, the thermal-mechanical “oscillations intensity” on the keyhole boundary is weak, and the molten bridge can stay force balance in the critical state of being penetrated, so the weld pool can maintain stable. When the VPFs are less than 33, the “oscillations intensity” is strong and the force balance of the molten bridge is broken, and the weld tends to be cut. If the VPFs are more than 83, the weld pool is unstable due to the broken force balance of the molten bridge although the “oscillations intensity” is weak. Finally, the accuracy of the developed model was verified by the comparison of the fusion zone, the keyhole penetration time and keyhole dimensions on the backside of the weld.