In view of the low concentration of influent water, which affects the emissions reduction potential and economic benefit of sewage treatment plant, taking Kunming as an example, a simulation model of urban drainage system based on system dynamics was set up. According to the targets of the 13th Five-Year Plan of Kunming, this paper designed four emissions reduction scenarios based on 2015 as the base year, and then simulated and predicted the emissions reduction potential and cost and benefit of sewage treatment plants under different scenarios in different periods. The urban drainage network reconstruction and strengthening management was very important to improve the efficiency of sewage treatment plant and fully tap the potential of pollutant treatment. Compared with the benchmark scenario, the reduction of COD in the short-term and long-term increased by 314772t and 389126t, respectively; the reduction of NH₃-N in the short-term and long-term increased by 24223t and 27234t, respectively; the cost-benefit ratio of the sewage treatment plant in the short-term and long-term reduced by 0.2 and 0.17, respectively. Emissions reduction potential was fully tapped and the economic benefit is obviously improved.