<p>The transient characteristics of transpiration cooling are important for hypersonic flight vehicles. This paper presents a two-dimensional transient semi-mixed model (SMM) developed to numerically analyze the two-phase flow and heat transfer performance of porous transpiration cooling during the startup process. The study investigates the transient behavior of porous transpiration cooling, including temperature and saturation evolution, as well as variations in evaporation mass. Additionally, the effects of Mach number (<i>Ma</i>) and porosity on transient transpiration cooling are examined and analyzed. The results reveal that the startup process of transpiration cooling progresses sequentially through three stages: an initial temperature rise (stage I), a constant temperature phase (stage II), and a second temperature rise (stage III) for the aerodynamic thermal surface. The durations of stages I and II are relatively short compared to the entire startup period, with the dry region moving downward until reaching an equilibrium depth in stage III. At higher Ma, stage II disappears, the inlet mass flux is insufficient for early evaporation, and the equilibrium depth of the dry-wet interface increases. Additionally, for a porous structure with higher porosity under the same driving force, the inlet mass flux increases, prolonging the duration of stage II. This, in turn, reduces both the equilibrium depth of the dry-wet interface and the overall temperature of the porous structure.</p>