Abstract: In order to achieve the goal of full coverage, full collection and full treatment of sewer system, many cities have carried out investigation and renovation projects of drainage network. The project of sewage quality and efficiency improvement represented by “Yangtze River Protection” pointed out that the pre-diagnosis of pipe network should be carried out before the comprehensive investigation and renovation work. In order to solve this problem, a pre-diagnosis method based on extraneous water quantity analysis is proposed. This paper introduces the purpose, significance, diagnosis content and technical route of the pre-diagnosis method, and comprehensively uses the water balance method, chemical mass balanced method and the method based on flow measurement to diagnose the sewage network of a southern city in the “Yangtze River Protection” project. The diagnosis results show that: the overall proportion of extraneous water in the sewage system of a southern city is about 16%; but the extraneous water in some catchments is more than 50% in dry days; the rainwater inflow in rainy days is serious, and more than half of the catchments are more than 30%. The pre-diagnosis of the drainage network has certain guiding significance to the investigation and renovation of the pipe network.