Urban vitality is an important reflection of a city’s development potential and urban quality. This study used exploratory spatio-temporal big data such as social media check-ins to portray the spatio-temporal evolution of urban vitality at the subdistrict scale in Changsha, a city in central China, from 2013 to 2021, finding that urban vitality in Changsha exhibited central agglomeration and outward circling expansion over time, and then we used Geodetector and spatial regression analyses to explain the interactive effects and spatio-temporal heterogeneity of the spatial elements of subdistrict form, subdistrict function, and subdistrict economy on urban vitality. The results show the following: (1) The subdistrict form and subdistrict function dimensions had a significant effect on urban vitality, and the effect of the economic dimension of the subdistrict was not significant. (2) The interaction effect of the density of entertainment and leisure facilities and the density of business office facilities in subdistrict function was the dominant factor in the change of urban vitality. (3) Under the spatio-temporal effect, land use diversity and park facility density had the strongest positive effect on urban vitality; road density and shopping facility density had the weakest effect. The study aimed to provide a reference for the optimization and allocation of spatial elements of subdistricts in sustainable urban development and urban renewal, in order to achieve the purpose of urban vitality creation and enhancement.