Underwater wet welding technology has been widely used in recent years. Improving the quality of underwater wet welding is the focus of many pieces of researches. The components of arc plasma of underwater wet welding directly affect welding stability and welding quality, but there has been little research on the composition of underwater wet welding arc plasma, there is a lack of diagnostic research on the composition of arc plasma from underwater wet welding by spectral spectroscopy. Through the research on the underwater wet welding process, the underwater wet welding experiment platform was built, through the arc spectroscopy diagnostic system, the obtained arc spectrum was diagnosed and analyzed, and the main elements considered in calculating the arc plasma composition were determined. Based on the spectral diagnosis results, the dissociation and ionization process of the arc bubble component in underwater wet welding were further analyzed, and 18 kinds of particles considered for the calculation of the underwater wet welding arc plasma component were determined. Based on the calculation of the partition function, the Newton iterative method is used to solve the equations consisting of the Saha equation, the charge quasi-neutral and the equation atom conservation equation, the number density of each particle is obtained, and the curve of the number density of each particle as a function of temperature is plotted. The calculation results show that the reaction occurs in the underwater wet welding arc plasma in different temperature ranges, and the main particles are different. At low temperatures, underwater wet welding arc plasma is mainly composed of unionized molecules, atoms and low-value ions with low ionization energy, as the temperature increases, the dissociation reaction and ionization reaction continue, and the ions of high valence state are continuously ionized. The tendency of different particles to change with temperature is also different. The calculated results show that the trend of each particle changing in different temperature ranges is consistent with the spectral diagnosis result. The determination of the plasma composition lays a foundation for the study of the arc of the underwater wet method from the mechanism layer, and provides a theoretical basis for further research on the thermodynamic properties and radiation properties of the underwater wet welding arc.