As the problem of water resources shortage and water pollution was intensifying day by day, it is of great significance to maintain river ecological flow and to ensure that the water quality does not deteriorate because of stagnation by reusing deeply disposed tail water of sewage plant as landscape water. How to plan the reuse of water and the location of the outlet to achieve economic, social, environmental and other benefits has become a problem worthy of study. This paper took Penglang sewage treatment plant of Kunshan City as an example, the deeply processed tail water was reused as landscape water in Xiajia river. Based on the three-dimensional unsteady water quantity model of Xiajia River constructed with environmental fluid dynamics model(EFDC), and considering the benefits in economic, social, environmental and other aspects, the comprehensive benefits of tail water reuse under 6 design scenarios were studied to determine the optimal operation scheme of the sewage plant. The results show that the when the water outlet is chosen to be 4. 5 km from Taicang Tang river with the drainage direction to the Wusong River and back flow is 34 000 m³/d, it is the best scheme that finally can be used to reuse the tail water of Xiajia River in Kunshan City, judging from the point of maximum economic, social and environmental benefits. In a word, the application of tail water to landscape water can effectively improve water exchange, the position of tail water flowing into the river from Penglang sewage treatment plant and back flow amount can directly affect the water exchange cycle and water quality of Xiajia River.