How to rapidly and effectively monitor the distribution and intensity of the urban thermal field in island-type cities, especially for understanding the impact of human activities on the urban thermal environment, is an urgent need to address environmental issues. Satellite remote sensing and geographical information system (GIS) techniques are used to retrieve the land surface temperature (LST), analyze the urban heat-island (UHI) effect to clarify its spatiotemporal characteristics and evolution rules, and clarify the impact of human activities on the thermal environment of an island-type city. The validity and applicability of the proposed method are demonstrated experimentally on Zhoushan Island, China. (1) The surface temperature of Zhoushan Island gradually decreases from the surrounding to the interior, high-temperature areas are mainly distributed in highly urbanized coastal areas with frequent human activities, and low-temperature areas are mainly distributed in interior regions with high vegetation coverage. (2) During the study period, the proportion of heat-island areas increases gradually, and areas with high heat-island levels are mainly distributed in the highly urbanized areas along the coast. (3) The proportion of heat-island of different levels changes negligibly, while moderate-temperature areas evolve into higher level heat-island areas. (4) LST and the UHI effect are negatively correlated with vegetation coverage and positively correlated with the density of urban buildings. The results of this study provide data support for urban planning and are of great significance for the sustainable development of sea island cities.