The article studied the conveyance capacity of siphon pipeline and observed the gas-liquid two-phase flow phenomenon under different heights of installation. It measured the flow of water level difference under different installation height corresponding and analyzed the change rule. The study found that with the increase of installation height, the gas-liquid two phase flow in siphon tube transforms from bubble flow into air mass flow.Through analyzing the reason it is found that in installation height hs equals to 2 m,the decrease rate of flow and area are equal, which shows that when the gas liquid two phase flow area is less than the liquid flow in full flow area is a major cause of the decrease of flow; when the installation height hs is greater than 2 m, the difference of flow reduce rate and area decrease rate is larger,which indicated that the decrease of flow is not only related to the area change, but is also related to the frictional resistance coefficient along the path. The increase of gas concentration makes pipe pressure drop increase, the increase of pressure drop causes the pipe resistance to increase, thus result in the increase of on-way resistance coefficient and the decrease of flow.