The coordinated development of population and economy is a goal that is easier to achieve in the development mode of urban agglomerations. The driving factors that affect population agglomeration and dispersion in urban agglomerations are similar to cities in some aspects: they can be divided into internal driving factors determined by resource environment and external driving factors dominated by policy guidance and public service level. Under the joint action of the market and the government, internal and external factors form two mutually influencing driving paths through the inner chain and the outer chain respectively, which jointly affect the population gathering and dispersion of urban agglomerations. Some factorsare different from that of cities: cities within urban agglomerationscan exchange and replace elements through “borrowed size” and “borrowed function”, making it easier to overcome internal and external driving barriers and achieve higher overall efficiency in the allocation of capital, human resourcesand other elements. Based on the analysis of the laws and driving factors of population convergence and dispersion in urban agglomerations, a mathematical model is constructed using the Production function, introducing resource and policy elements, and a simulation analysis is carried out with the “one core and two wings” structure in the capital metropolitan area as an example to explore the strength and realization path of each driving factor on population convergence and dispersion. The key to achieving population dispersion and succession is to improve the substitutability and driving elasticity of elements between cities. The exchange and substitution of elements between urban agglomerations can be achieved through cooperation mechanisms (borrowed size). On this basis, we propose that the rationalization of population quantity and distribution in urban agglomerations should be promoted by correctly understanding the connotation of population carrying capacity and urban functional positioning in urban agglomerations; taking the high-quality development strategy as an opportunity, strengthening the role of the market, and driving the rational distribution of population between cities through the rational layout of industries; reforming the management system, reducing the barriers of provincial boundaries and registered residence to promote the equalization of public services and infrastructure within the urban agglomeration, and fostering the formation of population gathering and dispersion that matches the functional positioning of the urban agglomeration and the cities in the agglomeration.