Global warming and intensive human activities have exacerbated the phenomenon of urban heat island (UHI). While studies on UHI mitigation strategies mainly focus on urban green space, ignoring urban blue space has prominent cooling effect with specific heat capacity and water evaporation. As a megacity in China, Beijing has frequent extreme heat events, how to optimize urban blue space to maximize its cooling benefits remains unclear. Taking Beijing as the case area, this study quantified the waterbody cooling benefits by indicators of cooling intensity, extent, efficiency and threshold value of efficiency (TVoE), identified the contribution of driving factors to cooling intensity in summer from four categories of blue, green, grey, and socio-economic. The results observed that the cooling effect of urban blue space has the characteristics of spatio-temporal heterogeneity, the mean waterbody cooling intensity (WCI) was 4.94 °C in summer and 1.29 °C in winter. The most prominent WCI driving factor was total building height in summer. For combined relationships affecting WCI, urban grey and socio-economic factors were proved as the most positively driving factors. The TVoE values of waterbody patch sizes were 1.01 ha, 0.60ha and 0.47ha in three different urbanization levels as high, medium and low. The results indicate preferable cooling benefits can be obtained through the planning and optimization of blue space, especially in dense built-up areas.