Abstract: Currently, legal research pays little attention to legal concepts and legal existence. Legal fiction not only has methodological significance but also ontological meaning, which can be used to explain the nature of legal concepts and legal existence. Nominalism holds that fictitious entities are placeholders that need to be reduced to real entities, denying the real existence of fictitious entities and ignoring the intrinsic dimension of legal concepts. Moral (legal) fictionalism argues that the "queer entities" referred to by moral (legal) concepts do not exist, but for functional purposes, it is necessary to pretend to believe in their existence. Both nominalism and moral (legal) fictionalism adhere to a single view of material facts, ignoring the existence of social facts and institutional facts. Institutional legal theory, drawing on the constructivist theory of social facts, proposes an ontological proposition that legal existence is a form of institutional fact. The transition from brute facts to institutional facts is a process of fiction, and institutional facts have a fictional structure. The doctrine of "analogy (fiction) of existence" also provides evidence for this. Legal fiction creates the legal world referred to by legal concepts, including legal subjects, objects, acts, and events.