The purpose of this study is to predict water influx and economic recoverable reserves accurately and simply in water-drive oil field. The calculation of water influx and economic recoverable reserves were based on the present production performance and Zhang’s water drive curve, and we have studied the characteristics of oil-water flow by using Welge equation. Then, according to water influx material balance principle, a new calculation model for water influx was established. This paper also proposed a method for determination of economic limit water-cut to predict economic recoverable reserves of oil fields. The economic recoverable reserves can be calculated on the basis of the economic limit water-cut. The case study shows that calculation process of the new method is simple and the results are accurate and practical.