Abstract: There exist inevitably some inefficient injection water after a long period of water injection in waterflooding oilfield. A concept of" effective production factor" is introduced in this paper through combining theoretical derivation with real data. Production factors at different pressure maintenance levels and fluid production rates and rational production factor at rational pressure maintenance level and fluid production rate can be determined through calculating the ratio of inefficient injection water in an oilfield unit. They provide theoretical basis for rational production proration and injection allocation in an oilfie1d.