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Urban parks are a major blue–green infrastructure in urban ecosystems, and they are widely regarded as being extremely effective in mitigating the urban heat island (UHI) effect caused by extensive urbanization and high temperatures associated with climate change. A scientific understanding of the cooling effects of urban parks can assist urban planning and decision makers in mitigating the UHI effect and improving urban sustainability. However, little is known about the cooling effects of parks from an accumulation-impact perspective resulting from spatial continuity. In this study, 31 urban parks in Fuzhou, China, were identified using Landsat data, and the land surface temperature was calculated using the radiative transfer equation (RTE) algorithm. Two accumulation-impact cooling indices, the park cooling intensity (PCI) and the park cooling gradient (PCG), and two maximum-impact cooling indices, the park cooling area (PCA) and the park cooling efficiency (PCE), were then used to explore the park cooling effects. The park area and park perimeter were found to be positively and significantly correlated with the PCA, PCI, and PCG and negatively and significantly correlated with the PCE. The results showed that 61% of urban park areas were situated within “cold-spot areas” with respect to the land surface temperature. A ward system cluster analysis showed that the 31 urban parks could be classified into three cooling capacity bundles based on the four normalized park cooling indices, each of which exhibited different cooling effects. The concept of the threshold value of efficiency (TVoE) based on the park cooling gradient was then calculated to determine the optimal park size. The TVoE was determined as 1.08 ha, which implies that urban park planning should consider designing urban parks of this size because they provide the most effective improvement in urban thermal comfort. These findings are valuable for providing a comprehensive understanding of the cooling effects of urban parks and providing implications for sustainable urban planning and design.