Based on state tree structures, supervisory control of preventing electric misoperation system is developed. A preventing electric misoperation system is an important part of substation. It guarantees that each operation of primary equipment in a substation is safety and reliable. The operating targets of preventing electric misoperation system are circuit breaker, switch and ground switch, which are discrete-event dynamic components that are event-driven and have discrete event behavior. Owing to the inherent discrete switching nature, in this paper we model the plant and specification as state tree structures. By using the STSLib software, the control logic (supervisor) is synthesized. In this paper we illustrate the detailed procedure of this approach and verify that the synthesized control logic (supervisor) is feasible.