To achieve the minimal operation cost of microgrids(MGs), a distributed economic dispatch method was proposed, in which a two-layer model was used. The upper layer is a communication network for information transmission, while the lower layer is a MG for power transmission. The communication network consists of two sub-networks, one of which is composed of all agents while the other is only composed of controllable agents. Then construction rules of communication network was established, and a general method was systematically given to derive distributed control laws from any communication networks. The distributed control laws derived from the first sub-network ensure the supply-demand balance, while the distributed control laws derived from the second sub-network minimize the cost of power generation, thus realizing the consensus of incremental cost of all the controllable DGs via the iteration of controllable agents. Based on equal incremental principle, the minimal operation cost of all controllable DGs was obtained. Finally, two simulation cases were designed to evaluate the performance of the method. The simulation results show that the proposed method can realize the minimal operation cost of MGs when the loads and renewable resources fluctuate widely.