A method for designing the auxiliary ventilation system for long drivages with multiple fans spaced in series in long duels is proposed based on the criterion that no recirculation takes place between the air flowing in the duct line and the surrounding air, A recurrence method is also given to calculat the leakage and the resistance lose of the duct taking into account of the relationship among the pressure, resistance and the leakage of the duct. The calculating results show that the present method is belter than the other methods.