A new plan formalism method and a plan knowledge graph concept. Compared with Kautz s formalism used widely in plan recognition, this method is simpler and more direct. Because a concept of supporting degree is introduced into plan knowledge graph, the recognized plans are changed reasonably as new evidences collected. A plan recognition algorithm based on knowledge graph presented in this paper can change the plan recognition problem into the graph searching one. This method not only prompts efficiency but also gives the same result as Kautz's.