According to the deficiency of present complicate pipe network’s calculating methods in smoke control system, a calculation model of complicate pipe network in buildings smoke control system is established here based on the theory of ventilation network. Graph Theory is used to obtain the model of pipe network in a smoke control system, air leak branch is applied to deal with air leak channels between pressurization area and non-pressurization area in a positive pressure smoke control system, and the equivalent air resistance of air leak branch is also calculated. Based on the equations of nodes flow balance and loop pressure balance, the calculation model of complicate pipe network is built to obtain the distribution method of air flows in a pipe network. The concept of fixed flow branch is introduced here to compute the total pressure drop and airflow regulation parameters of a pipe network. The research work in this paper perfects the calculation methods of complicate pipe network in a building smoke control system.