Based on multiphysics coupling of electric field, magnetic field, temperature field and flow field, a user-friendly integrated model of plasma arc and workpiece was established in this paper. It can accurately represent the multiphysics interactions in plasma arc welding but greatly reduce calculation time and cost, since it need not use Volume of Fluid (VOF) method to calculate the gas-liquid two-phase interface. The model calculates a precise arc pressure distribution, and then it acts on the workpiece as a source term. Meanwhile, the hot arc conduct heat to workpiece through the interface so that both heat and pressure force can transport from the arc to the workpiece. Temperature, velocity, current density and electromagnetic force of the arc and workpiece during the welding process are obtained, and the formation and evolution mechanism of weld pool are revealed. Welding experiment is carried out and verifies the model's accuracy. Finally, influence of different process parameters on heat flux and arc pressure is analyzed by the model to provide theoretical guidance for the optimization of welding quality.