The management of subway tunnel fire smoke is an important component in city public safety. In this paper, on the basis of theoretical analysis of the main factors affecting fire smoke flow in a subway tunnel, the ventilation and exhaust system were considered as a whole and the effects of buoyancy and thermal resistance were introduced. Further, a mathematical model was established to express the relationships between the critical environments, critical velocities, air speeds, smoke speeds and fire intensities. The results can provide instructions for tunnel smoke managements and emergency handling.