Abstract: Considering the weld geometric characters of keyhole plasma arc welding(K-PAW), a suitable and adaptive combined heat source for numerical simulation is developed, i. e. at the transverse cross-section and along the workpiece thickness direction, the double-ellipsoidal volumetric heat source acts at upper part of the workpiece while a linearly-increased peak value of heat flux in gaussian cylinder mode exerts at lower part of the workpiece. Based on the developed adaptive combined heat source model, the welding temperature field of 6 mm thickness stainless steel is simulated by SYSWELD. The predicted weld geometry and fusion line locus at cross-section are in good agreement with the experimental measurement. This demonstrates the suitability of the combined volumetric heat source mode.