Abstract: Since the Reform and Opening-up, large-scale population migration and rapid demographic transformation have reshaped the spatial pattern of China’s population distribution. Identifying the characteristics of population concentration and dispersion is crucial for revealing the spatiotemporal evolution of population distribution, thereby providing scientific evidence and decision support for regional planning, guiding rational population mobility, and optimizing population layout. Based on county-level census data, this study quantitatively evaluates and compares the evolution of China’s overall population concentration–dispersion pattern and the spatiotemporal variations in population agglomeration. The results show that from 1982 to 2020, population concentration at the national scale accelerated gradually, while aggregation at the provincial scale exhibited substantial intensity and pronounced regional disparities. The relative spatial pattern of population concentration remained largely stable, although the proportion of area and population within different concentration zones changed to some extent. The evolution of population agglomeration followed a spatiotemporal process that shifted from slow to rapid, from points to lines and areas, and from coastal to inland regions. Urban agglomerations played a crucial role in shaping the process of population concentration and dispersion. Therefore, the assessment of population concentration–dispersion and its evolution must be conducted within specific regional and spatiotemporal contexts.