Traffic flow in cities is increasing rapidly as cities modernize. The problem of parking is getting worse and worse due to insufficient parking lots in large cities, and people are paying more attention to safety issues in buildings due to the development of underground parking floors. There are new problems for fire safety because a building’s functions and materials, structural type, size, and supporting facilities are very different between the traditional buildings and buildings with underground parking floors. When a fire occurs in an enclosed underground parking floor, it is hard for people to evacuate due to incomplete combustion producing heavy smoke. In this work, we use the Fire Dynamic Simulator (FDS) to study the effect of a mechanical smoke ventilation system to retard the development of a smoke layer. The simulated results indicate that, although the installation of a mechanical smoke ventilation system would retard the deposition of a smoke layer, fresh air also drawn in increases the heat release rate of a car fire. Most importantly, a mechanical smoke ventilation system can extend the escape time for people during the initial stages of a fire.