Abstract: Informal institutions play an important role in the process of firms’ environmental innovation. Based on the existing articles published in domestic and foreign journals, this study reviews the definition and components of informal institutions, and analyzes their impact on firms’ environmental innovation and the underlying mechanism. It shows that the theoretical foundations for the influence of informal institutions on firms’ environmental innovation mainly include institutional theory, cultural dimensions theory, social identity theory and normative focus theory. Senior managers’ cognition, green dynamic capabilities and environmental strategy mediate the relationship between informal institutions and environmental innovation, while the variables such as corporates’ statistical characteristics (ownership structure and company size) and stakeholder, external environmental moderate the relationship. Finally, based on the findings, a research framework for the relationship between informal institutions and environmental innovation is constructed. It is suggested that future studies should examine the impact of different forms of informal institutions on environmental innovation, and explore deeper the internal mechanisms between the informal institutions and environmental innovation.