The application of underwater wet welding technology is becoming more and more extensive. Due to the special welding conditions, the welding quality in deep water should be improved. This paper simulates water depths of 0.3, 20 and 40 m respectively by building an underwater wet welding experiment platform and adjusting air pressure by pressure tank. After defining the arc initiation stage, the spectral information and voltage and current data of the welding arc initiation stage under three water depth environmental conditions were collected. When collecting spectral information, the delay trigger function of the spectrometer is used to collect the spectral data at the time of arc initiation of 5, 10, 15, 20 and 25 ms, respectively, and the collected arc spectral data is analyzed for diagnosis after sorting out. In the diagnostic analysis, the recognition results of each element particle were obtained by combining the relevant data of the NIST atomic spectral database and characteristic spectral lines. For the elemention state of high price state, the degree of excitation ionization will be affected by the change of arc temperature due to its large ionization energy. Therefore, it is not only necessary to identify and diagnose its components by spectrogram, but also need to calculate the number density of its components. Eighteen kinds of particles of arc plasma components to be considered in the calculation are determined by combining the spectral diagnosis information of underwater wet welding arc and the reaction process of underwater wet welding. The number density of the plasma component under three water depths is obtained by solving the equations composed of the Saha equation, dissociation ionization equation, quasi-neutral equation and gas pressure equilibrium equation. The variation law was analyzed to explore the influence of different water depth on the number density of welding arc plasma and its factors. The results show that the variation of particle number density is nonlinear under different water depth, and the variation amplitude of arc number density increases rapidly with the increase of water depth. As the water depth increases, the arc will be compressed, but the arc cannot be compressed indefinitely. The ionization of particles is affected by the temperature. The higher the temperature is, the stronger the ionization will be. However, when the temperature rises to a certain degree, each ionization has its ionization limit, and the particle number density will not increase indefinitely. Based on the calculation of arc number density, the mechanism of underwater welding arc is studied, which provides theoretical basis for improving welding stability and arc simulation.