Abstract: Based on the ablation and heat protection mechanism of ultra-lightweight quartz/phenolic composite, a multi-physics field model for the ablation of composite containing silica phase transition was established. The surface and backside temperature, pyrolysis degree, different layer thicknesses and pore pressure distribution of ultra-lightweight quartz/phenolic composite were predicted. The numerical simulation obtained the variation law of the thickness of the liquid layer of surface silica through calculation, and the temperature results predicted by the model are consistent with the measurement results in the ablation experiment. According to the heat flow analysis results of various heat transfer modes, it can be seen that the most important factors affecting the prevention/insulation mechanism are thermal radiation, thermal blockage and silica gasification. Aiming at the typical application conditions of ultra-lightweight quartz/phenolic composite, the heat flux densities between 0.5 MW/m² and 2.5 MW/m² were used as environmental input parameters to study the ablation behavior of ultra-lightweight quartz/phenolic composite. The results show that the surface ablation retreat of ultra-light quartz/phenolic composite increases with the increase of heat flux density; When the heat flux density is less than 1.5 MW/m², the thickness of the surface liquid layer increases with the increase of heat flux density. When the heat flux density is greater than 1.5 MW/m², the thickness of the surface liquid layer remains unchanged. This model provides certain guidance for in-depth research on the ablation mechanism of ultra-lightweight quartz/phenolic composite.