It is great important to accurately predict the water influx rate and cumulative water encroachment for the adjustment of oil and gas field development technology countermeasures. Combined with the principle of seepage flow mechanics, the mathematical model for calculating the water and water influx is established for a gas reservoir with an oil ring with an oil band. The model takes into account the changes of water invasion velocity caused by the variation of the seepage resistance in the oil zone. Through the argumentation and calculation, the rapid and practical algorithm is obtained. The example of the mathematical model for practical calculation established shows that the algorithm has comparatively high serviceability.