The purpose of the article is describing a tool for analysis and waterflooding management of large fields with a long development history. The developed tool contains a physical and mathematical model of the reservoir, which is a self-consistent solution of the equations of material balance, filtration and PVT correlation for each waterflood cell. To verify a simplified reservoir model, it is possible to forecast using historical well control parameters with their automatic adaptation. The paper describes the main stages of user interaction with the tool and its functionality. The results of a retrospective forecast for two different development reservoirs has been presented, the lacks and conditions of the model applicability were described. Integration with the tool on the basis of optimization algorithms was made and the results of multivariate calculations for the search for a scenario for developing a large oil and gas field with the best economic effect are presented.