This study investigates condensation and icing phenomena in high-pressure pneumatic reducing valves (HPAPRVs) using a mathematical model for moist air condensation flow under supersonic conditions. A numerical model based on nucleation and droplet growth theories was developed to characterize heat and mass transfer during expansion. The model was modified using different surface tension coefficients and validated against experimental data obtained from a Moses–Stein nozzle. Condensation characteristics and flow parameters were analyzed by comparing single-phase and non-equilibrium condensation models. The effects of valve opening on supersonic condensate flow behavior and the morphology of condensed droplets upon wall impingement were examined.Results indicate that the gas flow through the valve is adiabatic and isentropic, accompanied by a positive Joule–Thomson effect, which leads to rapid expansion, sharp decreases in pressure and temperature, and supersonic flow velocities. Intensive nucleation and droplet growth occur near the valve outlet, forming a primary condensation zone. Both models predict supersonic flow exceeding Mach 1.9 in the outlet section; however, the single-phase model yields higher flow velocities and a downstream shift of the diffusion point by approximately 2.1 mm, indicating that condensation phase change reduces the pressure reduction performance of the valve. Smaller valve openings result in stronger and more numerous vortices near the valve port, enhancing fluctuations in the liquid-phase mass distribution. Larger openings facilitate the solidification of supercooled droplets into ice crystals, but excessively large openings slightly reduce freezing efficiency, resulting in more droplets remaining in supercooled or unfrozen states. The present model provides valuable insights for structural optimization of HPAPRVs and further investigation of icing mechanisms.