The invention relates to a measuring and calculating method of the water level control improved efficiency of cascade hydropower stations. The method includes following steps: calculating a water level control target interval of a reservoir; calculating a practical generating capacity; calculating an examination electric quantity; determining a value of the responsible abandoned water power; calculating the storage capacity differential electric quantity; and calculating the water level control improved efficiency according to the practical generating capacity, the examination electric quantity, the responsible abandoned water power, and the storage capacity differential electric quantity. The invention also relates to an optimization dispatching benefit evaluation system of basin cascade hydropower stations in order to realize the measuring and calculating method of the water level control improved efficiency of the cascade hydropower stations, and the system specifically comprises a client layer, a service layer and a data storage layer. According to the method and the system, the problem of large influence of the water level at the beginning of the year on a hydropower utilization increase rate traditional algorithm is solved, and the unreasonable condition that the dispatching of the power stations often operate in a dead water level according to a design dispatching chart when the water level is low at the beginning of the year and the inflow is low is greatly improved.