The computation of coordinator in blocking decentralized supervisory control of discrete-event systems based on state tree structures is studied in this paper. This coordinator deals with the problem of system blocking(e.g. because of deadlock) that caused by the decentralized supervisory control. We take the reachable state set of the closed system as the specification and the closed system as the controlled plant, then synthesize the optimal supervisor of such control problem. We show that the resulting optimal supervisor is the coordinator as we want. The symbolic algorithm of computing the coordinator is presented. Based on this symbolic algorithm, we can get the coordinator easily even when the state size of the controlled plant is up to 1010. Finally, we give an example to illustrate our method.