There are mixed water supply zones between different water plants in multi-source water supply areas, where the hydraulic water quality conditions are not stable, and water quality problems such as excessive iron release and low residual chlorine are easy to occur. The mixed water quality risk area can be predicted by using the spatial modified interpolation method of characteristic water quality index. Characteristic water quality index refers to the index with distinct identifying characteristics, which are distributed regularly in the water distribution system and have large differences in the finished water of different water plants. According to its spatial distribution, the separate water supply area and mixed water supply area of each water plant can be determined. However, there are great differences between the distribution of pipe network and the geographical space, so the correction coefficient should be introduced before interpolation to correct the characteristic index. In this study, the mixed water supply zone of H city was predicted. Results show that the spatial modified interpolation results of the characteristic water quality index conductivity and total iron concentration of H city were basically consistent, and accorded with the results of hydraulic model simulation. Online monitoring points were set in the predicted mixed water supply zone, and frequent changes in flow velocity and flow direction were observed, which conformed to the hydraulic characteristics of the mixed water supply zone. It indicates that the prediction results are correct and the method is feasible.