Abstract: The electric arc is formed in the ionized gas bubble in the underwater wet welding.Combined with the previous bubbles components determination, the main ionization and dissociation process in the bubble are analyzed.The calculation based on the potapov model was done for the underwater arc components at different water pressures and temperatures under the local thermodynamics equilibrium state.Its main theorical bases are the Dalton law of partial pressure, the law of mass action, the electric charge quasi-neutrality condition and the chemistry measurement equilibrium condition.The results show that with the pressure increasing from 0.101 3 MPa to 1.013 MPa and then to 10.13 MPa, the density of H, H⁺, O, C, O⁺, C⁺ is increased, but the average ionization degree is not influenced by the water pressure.