Abstract: In order to study the mechanism of arc plasma breakdown in deep water wet welding, a high-pressure underwater wet welding experimental platform was established. Spectral maps of the arc initiation stage at a depth of 40 m were obtained, and a three-dimensional numerical model of arc breakdown discharge at a depth of 40 m was established based on the PIC-MCC method and analyzed. The results of spectral diagnosis arc plasma temperature, electron number density and numerical model analysis were compared, and the rationality and correctness of the model were verified. Based on the main components of the arc plasma obtained from the arc spectroscopy, a study was conducted on the dynamic evolution process of the arc plasma in high-pressure underwater wet welding from the perspective of microscopic particles, and the dynamic distribution, number of particles, temperature and electron number density of the arc plasma were obtained. The results showed that H⁺, OH⁺ and O⁺ were mainly generated by the ionization collision between the electron and the background component water molecule, and the number of OH⁺ increased fastest, followed by H⁺, and O⁺ last, the number of OH⁺ particles was far greater than the number of H⁺ and O⁺ particles; During the collision between electrons and background gas, the energy shifted, and the kinetic energy of electrons moving to the dielectric layer of the electrode decreased, the ionization collision reaction between electrons and the dielectric layer of the electrode weakened until saturation was reached.