When the plasma torch and the work-piece move relatively in keyhole plasma arc welding(PAW), it is found that there is a keyhole exit deviation distance between the keyhole center point and the welding torch axis. Considering the keyhole deviation, studying the interaction of weld pool and keyhole is of great theoretical as well as practical significance to obtain the high-quality welds. In addition, the research can also provide guidelines to optimize the control method, on the basis of taking the backside keyhole deviation distance as the controlled variable. A combined volumetric heat source model is established to reveal the dynamic interaction between heat transfer process and keyhole evolution. Along the thickness of the test plate, a deviation parameter of the volumetric heat source center is set in the model, which describes the thermal effect of the keyhole deviation in the PAW. In the keyhole shape equation, the backward pushing force of plasma jet is taken into account, which reflects the influence the plasma jet on the weld pool. The thermal conduction process and keyhole evolution in PAW are numerically simulated. The predicted and measured keyhole exit sizes and keyhole establishment time agree with each other.