Taking a sewage treatment plant in south China as an example, the evaluation of water quality and efficiency was carried out according to the drainage zone. The results show that the main problem in the service scope is the incomplete construction and serious external water invasion of the sewage system, where the water infiltration rate accounts for 21% and 54% respectively in sunny and rainy days. Combined with the information of comprehensive drainage network investigation, the priority and severity of problems in each district are determined. The current focus of improving the quality and efficiency of urban system is to separate the clean water and the wastewater. It is suggested that we should pay attention to both water quantity and quality, both the whole and part in the evaluation of sewage system, so as to effectively identify the main direction for improving the quality and efficiency of the sewage system and formulate corresponding strategies.