Gas switches are key components of pulsed-power systems and play an indispensable role in nuclear fusion, particle accelerators, and vacuum electronics. However, electrode ablation can reduce breakdown voltage, increase trigger jitter, degrade stability, and shorten switch service life, making it a major obstacle to the development of gas-switch technology. Although electrode ablation has been investigated numerically, simulation remains difficult because it involves plasma-electrode interactions and phase transitions among solid, liquid, gaseous, and plasma states. To address these challenges, this study develops a multiphase-flow model incorporating plasma-electrode interactions and applies it to cathode heating, melting, deformation, and molten-metal motion.The investigation focuses on ablation mechanisms and the effects of plasma and electrode parameters. The results show that thermo-field electron emission and ion bombardment are the dominant processes. Thermo-field electron emission primarily determines the surface current-density distribution, whereas ion bombardment governs energy deposition and pressure loading. Their coupled action causes cathode heating and melting, and plasma pressure then displaces the molten metal, resulting in crater formation and droplet ejection. Simulations and theoretical analyses further demonstrate that ablation severity increases with the electron temperature and ion number density in the near-electrode plasma. Higher values enhance the energy flux and bombardment pressure, thereby promoting melting and material removal.The effects of electrode properties are examined quantitatively. By combining heat-transfer analysis with molten-metal detachment dynamics, quantitative relationships are established between ablation behavior and key thermophysical and fluid-mechanical properties, including density, specific heat capacity, thermal conductivity, melting point, surface tension, and viscosity. On this basis, a new ablation-resistance constant is proposed to characterize the resistance of electrode materials to thermal damage and plasma-driven material removal. This constant provides a theoretical basis for comparing materials and can guide electrode selection and experimental design.The contribution of this work is a unified multiphase framework that couples plasma-induced energy conversion with phase-change heat and mass transfer. The model offers a quantitative method for evaluating electrode ablation and clarifies how interfacial processes affect gas-switch performance. These findings can support the optimization of operating conditions, the selection of ablation-resistant electrode materials, and the development of gas switches with greater breakdown stability, improved operational reliability, enhanced insulation performance, and longer service life.Some of the research results have been published.