Behavior permissiveness is the most important criterion in evaluating the performance of a supervisor. For a special class of Petri nets, S3PR, this paper present a deadlock policy which can give a maximally permissive supervisor. By using iterative method, Siphons in a net system are distinguished by elementary and dependent ones, only elementary siphons are required to create a set of generalized mutual exclusion constraints (GMEC) expressed by the marking vector. Monitors based on the GMEC are added to the plant model such that the siphons in the resultant net are all invariant-controlled and no emptiable siphon is generated due to the addition of the monitors. This novel deadlock prevention policy can usually lead to a maximally permissive supervisor by adding a smaller number of monitors and arcs, the proposed policy can work excellently in appropriate size petri net system. Finally the manufacturing example is utilized to illustrate the proposed policy.