Urban thermal environment issues have become a potential threat to the urban ecosystem and residents’ health. Thermal environment issues in Changsha are becoming more serious with the expansion of built-up areas and the decrease in the area of blue-green spaces. In this study, the spatiotemporal evolution of blue-green spaces and their influence on the thermal environment of Changsha from 1988 to 2019 were assessed using Landsat images. The results show that the loss of blue-green spaces and the exacerbation of thermal environment issues in Changsha began around 1999. Over the study period, the total area of blue-green spaces decreased by 427.27 km2, corresponding to 16.5% of the study area, leading to an increase in the average land surface temperature (LST) of Changsha of 2.59 °C. During summer, the cooling effect of water is the highest among all blue-green space types, which is −1.29 °C, followed by cropland (−0.82 °C) and forest (−0.55 °C). However, the cooling effect of forest in other seasons is always greater than that of cropland. The cooling effect of blue-green spaces in spring, summer, and fall is generally greater than that in winter. Over the study period, the temperature increases caused by spatial changes from water, cropland, and forest to built-up areas are 1.39, 1.17, and 1.1 °C, respectively.