In order to solve the problems of large voltage fluctuation and long switching time when switching between grid-connected and off-grid modes under the traditional control strategy, and the need to boost the voltage when the parallel structure is connected to the medium and high voltage power grid, a series photovoltaic DC microgrid control strategy with unified grid-connected mode was proposed. Firstly, the on-grid and off-grid structure model of the tandem photovoltaic system is established. Secondly, the DC bus voltage is regulated by droop control and PI control, and the on-grid and off-grid mode is unified through the difference between the reference power and the maximum output power, so as to realize the bus voltage stability and maximum power tracking during the switching process. Finally, the stability of the system is proved by the small signal method, and the feasibility and stability of the proposed control strategy are verified by experimental results.