The distribution of population is of great importance to regional economic studies, which helps reveal the characteristics and the development trends of regional spatial structure. This paper applied the regional density function approach to study the concentration and the decentralization of population in the Beijing-Tianjin-Hebei Metropolitan Region, one of the largest extended metropolitan regions in North China. Besides, the polycentric regional density function is used to analyze the population growth patterns of the study region. Compared with the classic monocentric density function, the polycentric density function is more appropriate for modeling the modern metropolitan regions, like the Beijing-Tianjin-Hebei region, which usually presents a polycentric pattern. The estimation of the polycentric density function shows the concentration of population into the core urban centers during the 1980s, the coexistence of the concentration of population into multi-urban centers and the decentralization of population from the core urban centers during the 1990s. At the same time, it is shown that three different growth patterns for the urban centers have formed at different levels, namely, the dispersion pattern through decentralization, the dispersion pattern through growth and the concentration pattern. Finally, a dynamic varying parameter model is proposed to identify the determinants of the spatial dynamics of the population distribution and growth, which indicates that the concentration and the decentralization of population within urban centers are influenced by the size, the economic structure and the transportation facilities of urban centers and their changes.