Abstract: To overcome the subjective factors of fuzzy comprehensive evaluation applied in water quality，the water quality indexes were assumed to be normal-distributed random variables，then the probability was introduced and a practical membership function was constructed． The method with the new function was sensitive to show a distinct difference in the comprehensive water quality of different time or waters by calculating a comprehensive pollution index． By researching 9-year water quality on Hefei section of Chaohulake，the method is proven to be effective．