Plasma arc welding (PAW) is an important joining technology for plates with medium thickness because of the heat source characteristics, however, most models of PAW neglect the vaporization of metal. An axisymmetrical unified PAW model was developed by taking into account the influence of Fe vapor behavior from the molten pool surface as an anode in this work. The simulation region includes tungsten cathode, plasma arc, weld pool, keyhole and their self-consistence coupling using one set conservation equations. A viscosity approximation is used to express the diffusion coefficient in terms of the viscosities of iron vapor. The main physical properties of Ar plasma are set as function of temperature and mass fraction of Fe vapor and are updated every iterate step to reflect the influence of Fe vapor in real time. The process of keyhole formation in stationary plasma arc welding is simulated under welding currents of 150, 170 and 190 A. The transient production, diffusion and concentration in the plasma arc of Fe vapor were presented. The effects of Fe vapor on the plasma arc behavior and formation of weld pool and keyhole are studied. It was shown that the evaporation rate of Fe was greatly dependent on the temperature of the weld pool. Most Fe evaporates from the top part of the keyhole surface and little from the keyhole bottom. The diffusion of Fe vapor is accelerated in the radial direction and is prevented in the axial direction due to the effect of plasma jets flow and at last it tends to be confined to the fringe of the plasma arc closed to the anode. The mixing of Fe vapor in the plasma results in the increase of radiation losses and the decrease of current density of the arc plasma in the fringe, but it had insignificant influence on the arc center. The heat flux from the plasma arc to the anode is also affected by Fe vapor due to its influence on the plasma arc properties. It is found that the calculation result of the width of the molten pool becomes more accurate to consider the effect of Fe vapor.