Planning is a wider use of public management means and methods, at the same time, the role of planning can play a different role, there is a planning can play an important role and planning inevitable failure of the two opposite views. How to scientific understanding of the planning? What is the internal mechanism of planning ? This paper, starting from the subject  and the object of planning, established the analysis framework of "function- knowledge" based on the function of the main body of planning and the knowledge type needed by the planning  object. It distinguishes four kinds of planning types. The research tries to enrich the cognition of planning and make the planning "something is not". In order to ensure the actual effect of the planning, a new understanding angle is put forward。