Abstract: The essential characteristics of overlaying welding, the influence of the flow and content of protective air on energy density, the arc radial pressure distribution, the heat distribution of plasma arc, the combustion stability and static characteristic of negative polarity weak plasma arc are analyzed.The results indicate that the arc pressure of negative polarity weak plasma arc is homogeneous, the radial distribution graph of arc voltage, the radial distribution graph of arc current and the distribution area of cathode spot all have saucer shape, the energy of cathode spot takes up 50% than that of the effective energy, 90% of the current distributes in the annular area of internal diameter of 10 mm, and changing the flow capacity of protective air can control energy density. The catelectrode atomising action of negative polarity weak plasma arc can effectively improve the combine condition of overlaying metal and basal metal, which makes negative polarity weak plasma arc be one of the perfect thermal resource at dissimilar material overlaying welding.