Abstract: Based on the traditional technique of estimation of water influx into an oil (gas) reservoir,this paper gives solutions to the following problems by the application of the differential equation of material balance: 1) The application of differential equation of material balance in inside and outside boundary water injection; 2) A technique for the estimation of water influx into an undersatu-rated oil reservoir,an oil reservoir with a gas cap or a gas reservoir by the application of differential equation of material balance; 3) A technique for the estimation of water influx in case that multiple reservoirs are included in an integrated aquifer when the interference of different reservoirs are taken into consideration.Thus,a complete set of technique of water influx by means of differential equation of material balance is resulted mad the application of such a technique is generalized.