During its first five years of operation, the Asian Infrastructure Investment Bank (AIIB) is becoming more and more similar to traditional Multilateral Development Banks (MDBs) in terms of operational goals, business area, and environmental and social standards. Why has the AIIB, the newest type of multilateral development bank (MDB) initiated by an emerging economy, undergone institutional isomorphism? Based on the socialization theory, this paper argues that the institutional environment in which the AIIB is operating has a strong influence on AIIB’s institution-building, mainly through the coercive, mimetic, and normative institutional isomorphic processes. On coercion, the pressures from European donors, international credit rating agencies, and global civil society have resulted in the AIIB’s institutional isomorphism. On mimicking, the social uncertainty of the relationship between the AIIB and the Belt and Road Initiative and the technical uncertainty of infrastructure projects have triggered the AIIB’s institutional isomorphism. On normativeness, the similar educational backgrounds and working experience of the AIIB’s staff and active interactions among the MDB family members have caused the AIIB’s institutional isomorphism. The paper concludes that the international institutional environment might hamper emerging economies’ institutional innovation.