The flexible aggregation of microgrid cluster is a technology that uses flexible DC transmission technology to interconnect multiple microgrids into a whole system, which can well overcome the problems of low frequency oscillation, inrush current of interconnect, high and low voltage electromagnetic loop network brought by AC interconnection, and has highly extensibility and flexibility. In this paper, the method of the flexible aggregation of microgrid cluster is systematically studied. Various basic structures of the flexible aggregation is proposed. The hierarchical control system of flexible aggregation is constructed, containing the optimized scheduling layer, coordinated control layer and sub-microgrid autonomous management layer. The aggregated and grid-connected mode, aggregated and islanded mode, disaggregated and grid-connected and disaggregated and islanded mode and corresponding switching control strategy are proposed under different operation scenarios. Finally, the flexible aggregation simulation model is built by PSCAD/EMTDC simulation platform. The simulation results show that the flexible aggregation can show reliability and stability under different operation modes.