Abstract: The water-cooling and magnetic field were used in the process of Fe based alloy plasma surfacing. The parameters of water cooling and magnetic field were adjusted during surfacing. The microstructure of surfacing layer was detected for analyzing the thermodynamics characteristics of iron-based. The results show that, the magnetic field and the forced water-cooling can change the distribution and morphology of hard phase in the surfacing layer. The external austenite transformation barrier and critical nucleation radius is decreased by magnetic field, while the phase transformation driving force increasies, so the nucleation rates of hard phase can be improved. The easy axis of magnetization of Cr₇C₃ hard phase is the preferred growth direction.