In a DC microgrid composed of multiple interconnected Distributed Generation Units (DGUs), the input or exit of the DGU makes the microgrid structure uncertain. To solve this problem, a Robust Model Predictive Control (RMPC) strategy based on a Tube invariant set is applied to the DC microgrid, with off-line design and online real-time control, so that the state of the DC microgrid with uncertain structure always converges to a terminal interference invariant set, and the bus voltage of the DC microgrid is stable when the DGU is put in or out. First, the state equation of the system is analyzed, and the linearized model is derived. Then, the robust model predictive controller is designed. Finally, the feasibility of the control method is verified by numerical simulation with Matlab. This work is supported by National Natural Science Foundation of China (No. 61963024).