Abstract: In order to deal with the uncertainty in the process of water quality assessment，interval numbers were used to represent the concentrations of pollutants in Bayesian water quality assessment model． The ranking-method of interval numbers was adopted to compare the results of water quality assessment． Then the interval Bayesian model of water quality assessment was established． The model was applied to water quality assessment of a lake，and the water quality of five sampling sites of the lake were monitored and assessed． The monitoring results showed that the water quality of the five sampling points was the V category of the water quality standard． The model of water quality assessment based on interval Bayesian model could more intuitively express the difference of the similar water qualities which attached to the same level of the water quallity， and also could avoid the influence of subjective factors in traditional method． So，the model established in this paper could provide a theoretical basis for the evaluation of different sampling sites belong to the same class of water quality．