This paper aims to explain how adjacent areas which around existing urban agglomeration integrate into established regional intergovernmental cooperation system. By introducing theories of institutional diffusion, this paper reveals and discusses the process and mechanism of institutional diffusion in the Yangtze River delta economic zone which expands from 16 cities to 26 cities in the past 11 years. Based on interlocal agreements, this paper constructs cooperation networks to reveal the process from preinstitutionalization, theorization, to widely diffusion, as well as the network structures and features in each stage. What’s more, this paper explains the main mechanism of institutional diffusion for the integration of adjacent areas into regional intergovernmental cooperation is the top-down institutional pressure, while There are also multiple institutional tensions that city governments are able to effectively resolve through three strategies: balancing, selecting, and dissolving when integrating into the regional intergovernmental cooperation system. By introducing institutional diffusion into regional intergovernmental cooperation, this paper also reveals multiple institutional tensions in the diffusion of regional cooperation, as well as the problem of formalization in the institutional diffusion.