Due to interconnection and power supply, multi-microgrid system can be composed of multiple microgrids in adjacent area. Based on researches of single microgrid phase-locked loop control, tie-line power and phase adjustment, this paper presents grid-connected and off-grid mode switching control strategies of multi-microgrid with series structure characteristic. For off-grid to grid-connected mode switching, the control strategies calculate mode switching time of off-grid to grid-connected in different cases of initial phase and energy storage frequency reference value. For grid-connected to off-grid mode switching, based on the active and passive mode switching, the control strategies adjust energy storage output power and tie-line power of the sub-microgrid. Simulation results verify that the control strategies can realize grid-connected and off-grid mode fast switching of multi-microgrid with series structure. This work is supported by National High-tech R & D Program of China (863 Program) (No. 2014AA052001) and Science and Technology Program of Guangdong Province (No. 2012B040303005).