Abstract: Megacities have formed due to rapid urbanization. As a result, the surface cover has rapidly changed, which changes the heat balance of Earth's surface and induces drastic changes in the thermal environment in megacities. With six typical megacities (Beijing, Shanghai, Guangzhou, London, New York, and Tokyo) across the world as study objects and multi-temporal Landsat remote-sensing images of the 1990s, the 2000s, and 2015 as the main data sources, this study compares the changes in the thermal environment among the six megacities and analyzes their causes. For each of the megacities, the surface temperature was determined through reversion using the universal single-channel algorithm and the urban heat island ratio index (URI) was calculated to quantitatively compare the spatial-temporal changes in the heat island effect during the study period. The results are as follows. From the 1990s to 2015, the URI values of Beijing, Shanghai, and Tokyo showed an overall upward trend, and while that of Guangzhou, London, and New York showed an overall downward trend. In 2015, Tokyo suffered the most serious urban heat island effect (URI=0.630), followed by Beijing, Shanghai, New York, and Guangzhou successively, of which the URI values were 0.617, 0.594, 0.555, and 0.530, respectively. In contrast, London had the smallest URI of 0.433. The megacities such as Beijing, Shanghai, Guangzhou, and Tokyo all considerably expanded throughout the study period. In these cities, the built-up areas and impervious surfaces increased by more than 500 km² and more than 370 km² on average, respectively in terms of area. They continuously spread outwards and occupied ecological land. Furthermore, green belts can not be formed between urban clusters. All these caused a significant increase in urban surface temperature and especially the significant aggravation of the heat island effect in new urban areas. In comparison, the thermal environment in the old urban areas was significantly improved through urban reconstruction. London and New York were not significantly expanded, where the surface temperature slightly changed. Therefore, it is necessary to pay attention to ecological philosophy, optimize the pattern of urban surface space, and improve the efficiency of ecological land in future urban construction.