As special and important landscape bodies, tidal inlets possess many coastal resources. Consideration of the maintenance and protection of coastline resources and the environment is a challenging research topic, i.e. how to reasonably and scientifically exploit and use coastal resources. In this study, multitemporal Landsat images were used to explore and quantify relationships between the intensity of human activities and coastline changes. Specifically, (1) the spatial location, length, and tortuosity of coastlines in Xiangshan Harbor (XH; Zhejiang Province, China) tidal inlets, from 1974 to 2014, were extracted from satellite images; (2) the coastline change indices and the intensity of human activities in the tidal inlet areas of XH were calculated and assessed; and (3) the relationships between coastline change characteristics and human activities during the past 40 years were explored and analyzed. The results demonstrated that, during the past four decades, as human activities have intensified, the natural coastlines in XH continuously shrank, whereas artificial coastlines increased and then gradually became stable. Various human activities have changed the morphology of local coastlines in XH, and the overall coastline tortuosity has been continuously reduced. The intensity of human activities increased from 1974 to 1998, and the intensity slowed after 1998. However, the impacts of human activities are extensive and almost cover the entire coastlines of the harbor. The correlations between the intensity of human activities and the overall length and tortuosity of the coastlines were significantly negative, which means that, as human activities intensified, the length and tortuosity of the coastlines decreased.