Cities are crucial in carbon reduction and carbon neutrality. However, the contribution of urban green infrastructure (UGI) to achieve these goals has been underestimated. Here, taking the Guangdong-Hong Kong-Macao Greater Bay Area (GBA) as an example, we develop a methodology incorporating fine-resolution UGI mappings and use the results from 2017 to 2022 to elucidate UGI patterns, trends, and carbon reduction capacity. The results revealed that: 1) Urbanization area increased by scarifying UGI, and the carbon sequestration capacity experienced a decrease and fixed 1.4% carbon emissions in GBA. 2) Based on the climate regulation from UGI, cities in GBA mitigated 29.5% of GBA’s carbon emissions. The total carbon reduction from UGI was 193.6 million tons in 2022. 3) By synthesizing global field measurement studies with meta-analysis, we found that the rate of UGI increased by 1%, and the air temperature decreased by 0.01°C. 4) With consideration of climate changes, we identified that increasing the coverage of UGI is more effective in strengthening the carbon emission reduction capacity than improving UGI quality. This study broadens the capacity of UGI in carbon neutrality, can effectively mitigate urban carbon reduction stress, and provides a novel perspective for urban carbon neutrality in China and elsewhere.