In conventional computational theory of cavitating flows, it is assumed that the pressure within the cavity or on the cavity surface remains constant and is equal to the vapor pressure of the clean water at 20℃ and at zero altitude. The cavitation is confused with the vaporization, and the effect of water quality on cavitation pressure characteristic is not taken into account. The effect of water quality on cavitation pressure characteristic was analyzed and the computational theory and method of cavitating flows that considered the influence of water quality was proposed. The theory is suitable for both the potential theory and two phase method for cavitating flow simulation. Finally the validation results for cavitating flows in a hydraulic turbine indicated the significant influences of water quality on the cavitating flow performance.