Abstract: There has been very little calculation of the composition of underwater wet welding plasma. The article collects and analyzes the arc spectral information by setting up an underwater wet welding experiment platform. Based on the results of the diagnosis and the study of the bubble composition, the particles considered in the calculation were determined. On this basis, Analysis of the equilibrium equation of underwater arc welding by arc plasma, Based on the law of mass action, five basic particles are selected, and other particles are represented by these five basic particles, substituted into the conservation equations, and the number density of each particle is calculated under specific pressure and temperature. This method and the traditional method of solving the Saha equation and other conservation equations to obtain different plasma particle density. The results show that the particles occupying the main components in different temperature zones have different effects on arc plasma. It can provide theoretical basis and basis for further study of underwater wet welding arc stability and welding quality, and can also be combined with spectral information for temperature calculation and particle determination in main temperature range.