Abstract: Based on the panel data of 191 cities of 12 urban agglomerations in China from 2005 to 2018, the synergistic effect of industrial agglomeration and urban population size on economic growth is examined. It is found that, on the whole, China's specialized agglomeration is not conducive to economic growth and the promotive effect of diversified agglomeration on economic growth has not yet been produced. With the expansion of population size, the effect of agglomeration economy is inhibited. Specialized agglomeration is suitable for "small and fine" urban development and diversified agglomeration is suitable for "large and comprehensive" urban development on the premise that the economic externality of diversified agglomeration is produced. In the Yangtze River Delta and Pearl River Delta urban agglomerations, the potential of industrial development is limited, and the expansion of population size is conducive to the promotion of economy by industrial agglomeration. The effective solution to real coordinated development of regional economy is a transition from agglomeration to balance.