Taking the Beijing-Tianjin-Hebei and Yangtze River Delta urban agglomeration as examples, this paper analyzed the difference and reason for the driving effect of the core city on the peripheral areas in urban agglomeration. The economic factors in the Yangtze River Delta urban agglomeration spill over to the peripheral areas obviously, while the economic factors in the Beijing-Tianjin-Hebei are opposite, Shanghai has driven the growth of its periphery areas, while Beijing’s effect is limited. The reasons for this difference include three aspects: the radiation capacity of the core city itself, the carrying capacity of the peripheral areas and freer flow of factors between them. Based on the above findings, we put forward relevant policy implications.