Oil and gas production from reservoirs is the result of drive of reservoir energy. Reservoir energy evaluation is a routine job during exploitation of reservoirs. Conventional evaluation parameters of reservoir energy are the dimensionless elastic production ratio and the pressure drop per recovery of reservoir reserve. However, the two evaluation parameters both give out logically inverse results, i.e. the reservoir energy is increasing with the depletion of reservoirs. In order to evaluate reservoir energy scientifically, this paper proposed energy index method, which is the ratio of reservoir pressure to limit bottom pressure of production wells. The new method is closely related to the lift style of wells, and can properly evaluate the reservoir energy status with the reservoir energy decreasing while depletion. The application result shows that the energy index is easier and more practical to use than the conventional method.