In urban areas, the increasing frequency of heavy precipitation caused by climate change has generated huge pressure on drainage systems, which could result in severe environmental or social issues if not adapted to. A critical step to enhance the adaptive capacity of drainage systems is to build an authoritative indicator-based framework for assessing the adaptive capacity. An assessment framework with three levels, including an index (Adaptive Capacity Index), five parameters (Economic resource, Information and skills, Technology, Infrastructure, and Institutions) and several indicators, was established after literature review and expert consultation. The Delphi method was applied to optimize indicators in order to form the applicable indicator-based assessment framework. Through two rounds of Delphi process, a consensus among experts was reached and was checked for consistency by statistical analysis. Eventually, eight indicators of adaptive capacity were determined with high consistency and an inherent linkage was revealed from the five dimensions (parameters) that contribute to the adaptive capacity of drainage systems. Overall, the construction of adaptive capacity of drainage systems is a systematic engineering, and the Delphi method proved to be an appropriate technique for framework building.