The formal reasoning of the fuzzy propositional modal logic based on plausibility degree is considered,and the description of the associated Kripke semantics is given. The fuzzy constraint is introdu ced and used as a basic expression, the set of reasoning rules based on fuzzy constraint is given and a formal reasoning system is established, and in which the notation of the satisfiability is introduced. The relationship between the fuzzy reasoning and the satisfiability of the associated fuzzy constraints set is studied, and the soundness and completeness of the fuzzy reasoning based on satisfiability are proved.