Abstract: Phase change transpiration cooling is regarded as a highly promising active thermal protection for near-space vehicles. However, there still remains a lack of mechanism research on the gas-liquid phase evolution and heat transfer within porous media during phase change. To solve this issue, an experimental approach integrating gas-liquid phase visualization, evaporation rate and temperature variation is adopted in this work, and a pore-scale study is conducted to investigate the phase change heat transfer mechanism within the porous media under high heat flux condition (50 kW/m²). The characteristics of gas-liquid two phase distribution, interface evolution, evaporation of coolant and heat transfer performance are analyzed, and the influence of glass beads particle size on gas-liquid two phase evolution, heat and mass transfer is explored. The results indicate that under high heat flux condition, the evolution of gas-liquid interface dominated by capillary force and gravity is the fundamental impact on evaporation rate and heat transfer characteristics within the porous media. As the particle size decreases from 2 mm to 0.5 mm, the connectivity of the liquid film significantly improves, the thickness of the two-phase region increases by 65%, and the duration of rapid evaporation and efficient heat transfer within the porous media is extended by 38%. This study provides theoretical foundation for the design of future high-efficiency transpiration cooling systems.