Original gas in place（OGIP） and water influx are the key parameters to determinate development strategy and production scheme for a gas reservoir. Traditional apparent formation pressure is common process for judging gas reservoir drive mechanism. However for those gas reservoirs weakly affected bywater drive, its pressure drop curve is a straight line in early period, easily to be misinterpreted as elastic energy driving, and its OGIP calculation results are also greater than using geologic reserve calculation method with water drive. TakingA gas reservoir in Sebei No. 2 gas field as an example, this paper applied apparent geologic reserves method under the situation of unknown water influx to estimate OGIP from mapping apparent geological reserve curves, and then backward calculated water influx by putting OGIP data into the equation. This calculation method is proved to be an effective approach for Sebei Gas Filed. Simultaneously, it also has positivemeaning to guiding the gas field development during the later period.