This paper develops a deadlock prevention policy for Petri nets that can model concurrent manufacturing assembly processes in flexible manufacturing systems (FMS). They can be modelled by a class of nets, namely G-systems. They are supervised to have the non-blocking property of the behavior, i.e., from any reachable state, a desirable state can be always obtained under supervision. Their deadlock situations in terms of insufficiently marked siphons can be characterized. The proposed approach is to make these siphons satisfy controlled-siphon property (cs-property) if the elementary siphons are properly supervised. Compared with the existing policies, the advantage of the method is that a much smaller number of supervisory monitors and arcs are added and iterative computing processes are avoided. Finally, an application of this technique to a G-system is presented.