While various methods exist for assessing urban rainwater and flood resources, there is a lack of targeted evaluation for the rainwater harvesting potential of areas equipped with sponge city facilities. This study employs the Yield Before Spillage (YBS) principle to design rainwater collection tanks for sponge facilities under different design return periods, conducting a specialized assessment of the rainwater resource potential in built-up sponge facility areas within the "Dongsheng-Kangbashi-Ejin Horo Banner" urban cluster. The results indicate that the collection potential follows the patterns of "wet year > normal year > dry year" and "Ejin Horo Banner > Kangbashi District > Dongsheng District." A rainwater collection tank designed for a 5-year return period (p = 5a) is more applicable to the study area. The sponge facilities in the study area achieve an annual runoff volume control rate exceeding 85%, effectively alleviating drainage pressure. The conclusions demonstrate that the YBS method can effectively assess the rainwater and flood resources of sponge facilities in arid regions. Tanks designed for the three different return periods all meet the rainwater retention requirements of sponge cities across various hydrological years. In arid areas, tanks designed for lower return periods are sufficient for harnessing rainwater collection potential, offering lower costs.