Combing the water-air stratified flow theory, a theoretical calculation model of the joint flood discharge and ventilation system of discharge and spillway tunnels in Rumei Hydropower Station is established to clarify the ventilation characteristics of the multiple tunnels. The variation patterns of negative pressure in each tunnel, wind speed and ventilation quantity change of each air duct with the height of free space above water in discharge tunnels and the cross-sectional size of air ducts are plotted and summarized. The results show that the negative pressure of the discharge tunnels presented the pattern of three partitions with the co-existence of minimum and maximum values, whereas that of the spillway tunnels showed no obvious change; the maximum value of wind speed and ventilation quantity of air duct occurred in the discharge tunnels, whereas the minimum value of the ventilation quantity of air duct occurred in the spillway tunnels. An overall consideration of the ventilation parameter changes in each tunnel can provide a design basis for the size selection of discharge tunnels and air ducts in the joint flood discharge system.