Rail transit hub is key node in urban transportation system. The study on properties of ventilation and smoke control system can improve response ability in case of fire and give ventilation and smoke control design guidance for underground rail transit hub. Take Beijing DongZhimen underground rail transit hub as research object, according to its air turbulent flow characteristics, introducing standard turbulence model based on the N-S equation. Build the computational fluid dynamics model for solving the flow field within underground rail transit hub. Through theoretical analysis and numerical simulation, take "Siamese model” results as the boundary condition and establish a mathematical model of rail transit hub with CFX. Then research fire smoke development and spread laws when the exhaust fan within underground rail transit hub delayed open.