Hot stamping of ultra high strength steel offers the possibility to reduce the weight of automobiles, while meeting the safety requirements. In hot stamping processes, the blank is hot formed and quenched in a water-cooled tool to achieve high strength. Hence, the temperature distribution of the blank during the process is very important for designing the tools with necessary cooling capability. In this paper, PAM—Stamp software was employed to build the hot stamping FE model of the B pillar reinforcement of passenger cars. Through the thermo-mechanical coupling analysis, the temperature distribution of the blank was obtained. The results show that the temperature in the holding area of the blank decreases quickly, whereas the side wall and bottom are cooled slowly. The inhomogeneous temperature distribution of the blank will cause the material unevenly flow during the forming process. Therefore, maintaining uniform temperature distribution and fast cooling of the blank are crucial for the cooling system of the tools.