Abstract: The implementation performance of forestry and grassland ecological construction projects is subject to the dual influence of formal and informal institutions. From institutional embeddedness theory and the social practice perspective, we systematically analyze the manifestations, generative logic, and operational mechanisms of informal institutions in forestry and grassland ecological construction projects. The research findings indicate: (1) Community traditional ecological knowledge, social cooperation networks, and ecological ethical values constitute the three main forms of informal institutions in forestry and grassland ecological construction projects, which are embedded in the ecological field, socio-economic field, and cultural field respectively. (2) Informal institutions evolve dynamically through the “internalization-creation-adaptation” behaviors of grassroots implementers under the influence of historical institutional legacies and path dependence. (3) Informal institutions affect project implementation through three mechanisms: supplementation, adjustment, and conflict. The supplementation mechanism manifests as functional filling of knowledge, organization, and norms; the adjustment mechanism embodies flexible resolution of objectives, rules, and procedures; and the conflict mechanism originates from structural tensions at the levels of values, rules, interests, and behaviors. Therefore, this study proposes constructing a “ formal-informal ” institutional synergy framework to promote the transformation of forestry and grassland ecological governance from project-based implementation to institutionalized construction, thereby achieving inclusiveness, adaptability, and sustainability of the governance system.