This paper presents a new mathematical model, semi-mixing model (SMM), to describe transpiration cooling with coolant phase change from liquid into vapor through two-phase process. The local heat exchange of fluid-solid within pores is considered in this model, and therefore it is closer to real transpiration cooling condition. The differences from the separated phase model and two-phase mixture model are that SMM can overcome the trouble of tracking phase change interface and avoid the inveracious numerical phenomenon, i.e., a thermal insulating layer occurs within the porous matrix. Using SMM, the corresponding numerical method is realized to simulate the processes of coolant moving, absorbing heat and evaporating within porous matrix. To validate SMM and the numerical strategy, an experiment is conducted. Using the validated SMM and numerical strategy, the effects of two-dimensional coolant injection rate and two-dimensional heat flux on transpiration cooling characteristics are simulated and analyzed. The simulations and analysis discover several interesting and valuable phenomena.