Abstract: Extraneous water intrusion in sewer networks poses significant risks to urban aquatic environmental health and public safety. This paper reviews the basic principles, applicable scenarios, research progress and deficiencies of quantitative diagnosis methods for extraneous water intrusion in sewer networks at home and abroad. The research shows that: though closed-circuit television (CCTV) inspection remains the primary method for precise source identification of such intrusions, its large-scale application is economically prohibitive. Quantitative diagnostic methods for extraneous water intrusion can rapidly pinpoint critical problem areas and core pipeline segments, enabling targeted CCTV inspections to achieve cost reduction and efficiency improvement. Water balance analysis, tracer-based mass balance analysis, and inverse modeling-based localization methods and other single quantitative diagnostic methods have a certain applicability but at the same time there are also limitations. By breaking through the technical bottlenecks of accurate flow monitoring, scientific screening of parameter tracers, accurate simulation and efficient inversion of external water intrusion, we have constructed a hierarchical diagnostic technology coupled with tracer-based mass balance analysis and inverse modeling-based localization, which is expected to promote the diagnosis of external water intrusion in the direction of normalization, high efficiency, traceability and intelligence, and to support the management of pipeline operation and maintenance efficiency. In addition, in the context of the industry focusing on improving the quality and efficiency of wastewater treatment, how to scientifically construct the risk evaluation method of external water intrusion into drainage networks to effectively support the decision-making of pipeline restoration and management, is also a difficult problem that needs to be focused on.