Abstract: In analysis of how the water quality changes with the change of water quantity, firstly we adopt correlation analysis to determine the relevance between water quality and water quantity, and then build the relational model of the initial water quality and the ultimately required amount of water, finally we test the model.In the process of engineering experiment, the highlight is that how munch water is required in Changshu water control to make the water under the Wangting interchange gates reach the standard.For the water quality under Wangting interchange gates is influenced by the water along the Wangyu River, we suppose that the initial water quality is measured by the initial water in all sites, and that when the water in each site along the Wangyu River reaches the standard, the total amount of water flow through the Wangting interchange gates is the required amount, so that we can guarantee the data used to modeling is reliable.In the process of modeling, considering the influence of seasonal changes and outliers, we respectively build different models based on correlation analysis, and then by comparison, we determine the final model, which has guaranteed the accuracy of our approach.