In order to understand the characteristic of the gas-liquid two-phase flow in siphon pipe, based on the homogeneous model in multiphase flow theory of ANSYS CFX software and RNG kεturbulence model, the process of gas-liquid flow for a siphon outlet pipe in pumping station was simulated. Results of simulation showed that the air sacs are easily generated on the shell of siphon pipe when the gas mixed into the water and it has an extremely adverse influence with the siphon finish. Meanwhile, flow rate of the incoming flow is the main factor impacting the siphoning time. At last, the relations between the siphoning time and flow rate for hydraulic driving gas and mixing gas-water processes, according with the power function relationship, were given.