Urban water environment treatment focuses on the source. Due to the unreasonable design and non-standard construction in the early stage and other historical reasons, the internal rainwater and sewage discharge in the current housing estates is disordered, which results in poor internal environment of the housing estates, and the municipal rainwater and sewage pipe networks and the sewage treatment facilities cannot give full play to the benefits. Under the background of the national and regional efforts to improve the water environment, taking a rainwater and sewage mixing reconstruction project in a housing estate of Shanghai as an example, combined with the situations encountered in the process of design and construction, the problems existing in the rainwater and sewage mixing within the housing estate are reviewed and summarized, and the relevant solutions are proposed.