Human's over exploitation of water resources seriously threatens water ecological security. Based on the idea of water ecological security, this paper puts forward the standard to maintain the regional water ecological health, i.e., concept of water interferential limit. Then, the theory that regional evapotranspiration is equal to ecological water consumptionwas demonstrated by the regional water balance equation, and the water interferential limit formula were derived. After that, based on the water interferential limit formula, regional real water interferential degree formula was constructed. When the real water interferential degree was equal or smaller than water interferential limit, regional water ecology was assured. Finally, two examples were shown to illustrate the practical applications of water interferential limit and interferential degree.