Abstract: The exponential computation complexity of the state reachability analysis makes it di±cult to apply forbidden state controller synthesis method to large plant nets. To solve this problem, a combined controller synthesis method is proposed for marked graphs. Firstly, the methods of designing observers are obtained. Secondly, the algorithm of calculating the maximally permissive control policy is proposed using the markings of the observers. The results show that the onlinecomputational efficiency of the controller is increased, because the state spaces to be observed is decreased in dimension number. Furthermore, the plant nets are not necessarily alive or bounded.