Abstract: In the drilling of well S45, which is on the edge of the Upper Suweiyi Formation in Dalaoba-2 structure, we did not find the gas-water contact, and took the sand bottom boundary of this well as gas-water contact in reserve calculation, which led to a big error and seriously affected the authenticity of reserves. This paper, based on the principle of capillary pressure, used reverse-hinking, and calculated the height of gas by gas saturation, and then determined the gas-water contact for gas reservoir. Using this method, we calculated the gas-water contact for the Lower Suweiyi Formation of Dalaoba-2 Structure, and it was nearly the same as the actual gas-water contact. We also calculated the gas-water contact for the Upper Suweiyi Formation, and then predicted reserve.