To solute the subjective problem of original information in searching accidental pollution source in a basin, the paper set up water quality model of non-persistent pollutants based on the search characteristics of accidental pollution source.It took Yibin section of the Yangtze river as research object,and got the complex degradation coefficient of COD by the scene sampling and indoor experiment. It simulated the diffusion concentration of the sewage outfall in the river reach.Through fitting the actual monitoring value, the validity of this model is validate.On the above basis, it built a method of getting original probability of accidental pollution sources by using error sequence and modified the original probability in search action.Therefore the original probability was transformed into posteriori probability. This result can provide an objective and scientific method for determining the target distribution function in the search model of accidental pollution sources in a basin.