Abstract: Through experimental research of closed gas reservoir depletion and waterflooding gas reservoir development,we studied the material balance and the law of water influx in the water-flooding gas reservoir.The simulation results for depletion of closed gas reservoir showed that there is a linear relationship between pressure and cumulative gas,which is consistent with theoretical formula.So the experimental system can simulate the development of actual gas reservoir.The simulation results for waterflooding gas reservoir indicated that there were some differences between experiment results and theoretical calculation results,which would be more and more prominent as water influx increase.The reason is that in material balance equation it did not take the effect of water influx into account on the trap forming process from reservoir and the generation of trapped gas.Based on the theoretical and experimental analysis,we did modification on the original material balance equation of water flooding gas reservoir,which takes trapped gas into account.The results from newly modified equtions matched experiment data very well.So the added item in the equition can describe the affecting degree of water influx to produce trapped gas,which have important significance in calculating water influx and predict development dynamics of water-flooding gas reservoir.