Current research on Urban Heat Islands (UHI) and Urban Cool Islands (UCI) primarily focuses on their microclimate mechanisms, with relatively little attention paid to public health and wellbeing. This study investigates the relationship between the UCI effect and public expressed happiness (EH) in Nanjing's central urban area, which has a resident population of 6.7 million. Using Landsat 8/9 satellite images and atmospheric correction, we derived land surface temperature (LST) for early August and classified the results to identify UCI areas. We then overlaid a 300 × 300m grid and conducted sentiment analysis on more than 760,000 geo-tagged tweets from the Sina Weibo platform using a pretrained natural language processing model. ANOVA and correlation tests were employed to examine links between UCI indicators and EH. Findings indicate that UCI areas have significantly different EH levels than UHI areas, and EH correlates negatively with LST. These insights support urban planning and health management efforts, contributing to sustainable urban development and improved resident well-being.