In order to analyze the characteristics of the ventilation system of a spillway tunnel, the balance relationship between the tunnel freeboard and the cross-sectional area of the ventilation was studied. The equilibrium relationship between the drag capacity of water, tunnel freeboard and air ducts were systematically analyzed based on a proven theoretical model for spillway tunnel with multiple air ducts. The variety of air pressure in freeboard and total ventilation was related under the conditions of fixed size of air ducts. A family of curves was plotted to reveal the equilibrium between the air duct and tunnel freeboard, in which three partitions and two extremum points were summarized. It’s suggested that the corresponding curves should be calculated and plotted when designing a spillway tunnel and its ventilation system. The design should be implemented within the range of the partition Ⅱ to optimize ventilation characteristics, which can ensure both the tunnel freeboard and the cost for air duct area.