Spatial planning is crucial for Sponge City (SC) construction; however, prioritizing SC sites at the watershed scale has not been fully explored. In this study, a multi-criteria decision model, considering demand and suitability of SC construction, was established by monitoring, model simulation and index calculation. This new model was then tested in a rapidly urbanizing watershed, Beijing, China and the priority of SC construction at both grid scale (1km 1km) and subwatershed scale was ranked. The results showed that the highest priority was found in emerging regions where urbanization is ongoing, and followed by urban core areas. In addition, six indexes were identified by clustering heatmaps as key factors affecting the priority of SC planning, including topographic index, water pollution index, pollution rate based on the state standard of surface water environment quality, urbanization planning, urban levels, and vegetation index, which could guide SC planning in data-lacking regions. The approach and findings in this study can not only provide helpful references for watershed managers and urban planners but also can be easily used in other regions.