Abstract: The RNG k-ε turbulence model and VOF gas-liquid two-phase flow model are used to numerically simulate the exhaust processof the air masses at the top of the large-diameter square pipe siphon outlet channel,and calculating the minimum hump average flow velocity varequired for humps with different aspect ratios to take away the air masses at one time and the average hump flow velocity v_b and dischargetime t_b required for air masses fragmentation to completely exhausted.The results show that as the hump aspect ratio decreases,the flow veloc-ity varequired to discharge the hump air masses at one time also decreases. When the hump aspect ratio decreases from 0.4 to 0.3,the corre-sponding va decreases by 11.6%,but the flow rate is still large. Under a smaller hump flow rate,the whole process of crushing and discharging the air masses is simulated. After a period of time,all the air masses can be broken and discharged,and finally a stable siphon is formed.Within a certain range,the longer the exhaust time,the smaller the flow velocity at the hump. When t_b increases from 120s to 180s,the cor-responding v_b decreases by 16.67%.This study is of great significance to the design of the hump section size of the siphon outlet channel andthe stable operation of the pumping station.