Abstract: Reaction infiltration is an important process for preparing ceramic matrix composites. The pore structure of carbon/carbon preforms is a key factor in the control of such processes, but experimental observation of pore structure evolution during infiltration is difficult. Therefore, in this paper, a multi-physical field model of melt infiltration-reaction-pore structure evolution was constructed using numerical simulation methods to study the pore structure evolution characteristics of carbon/carbon preforms. The results show that the pore structure characteristics of carbon/carbon preforms have a significant impact on the melt infiltration process. Contraction pores increase the infiltration height through dynamic curvature adjustment, while expansion pores cause a decrease in capillary force due to the retraction of the infiltration interface. At the connection of asymmetric pores, the narrow branches have a larger capillary force, resulting in non-uniform infiltration and the appearance of infiltration gradients. During the melt infiltration process, a reaction occurs with the carbon matrix to generate ceramic phases, gradually changing from interface reaction to diffusion transmission. The reaction rate decreases with the generation of ceramic phases and increases with the rise in temperature, eventually leading to the closure of the pore structure in the preform. This model can predict the pore closure time based on the initial pore size of the preform to precisely control the reaction infiltration duration, providing a theoretical basis for the optimization of the pore structure of carbon/carbon preforms and the control of the reaction infiltration process.