Abstract: Due to the environmental and formation pressure decrease restrictions, since 1999 water injection was re-applied after 8 years of pressure decrease development of Wumishan Reservoirs. If water injection was applied, what should be the perfect pressure level and the pressure restoration rate to maximize the output economically. To answer this question, numeric simulations was calculated for various injection/output ratios, liquid outputs and well patterns, to determine the appropriate (or rational) pressure levels and pressure restoration rate. Meanwhile, economic evaluations of every development program were computation to help to determine the best project. The practice of water injection showed that it was consistent with the conclusion of numeric simulations. The adopted development project was to inject water only when the reservoir pressures drop to its limit, and maintain at low injection/output ratio and pressure level to utilize natural energy of reservoirs completely.