Building climate-adapted cities is vital for urban security and sustainable development. Using 269 prefecture-level cities in China from 2011 to 2020 as research samples, this study investigated the effect of pilot climate-adapted city policy on urban sustainability and the underlying mechanisms through differential-in-difference modeling. The findings revealed that: (1) The pilot policy significantly boosted urban sustainability (coefficient = 1.057, P < 0.01) with a two-year policy effectiveness lag. (2) The policy had a significant positive effect in cities with low and moderate precipitation but no significant impact in high-precipitation cities; meanwhile, the policy effect was more pronounced in cities with higher economic development levels. (3) Human capital development and technological innovation were key pathways through which the pilot policy improved urban sustainability. This study confirms the pilot climate-adapted city policy's positive role in promoting urban sustainability and offers suggestions for better policy implementation.