Bus voltage stability is the key to the normal operation of DC microgrid. In a DC microgrid adopting droop control, zero-error regulation of DC bus voltage can't be achieved. When adopting double closed loop control, micro-sources can't adjust their output power according to their own capacity flexibly, thus leading to the low efficiency of the microgrid. To deal with this problem, a secondary voltage regulation system was designed, by integrating these control methods above. When the DC bus voltage deviates from the rated value, the primary voltage regulation is conducted by the droop-control system firstly. The DC bus voltage is regulated to the stable working range. Then the secondary voltage regulation is conducted by the vanadium redox battery (VRB) storage system, with the cooperation of the lithium battery in island mode or the grid-connecting converter in grid-connected mode. The DC bus voltage is regulated to the rated value. If a large power fluctuation occurs in this time, the DC microgrid returns to primary voltage regulation again. The processes above is repeated. The primary and secondary voltage regulations cooperate with each other, switch flexibly and achieve the efficient regulation of the bus voltage. The proposed voltage regulation system realizes the zero-error regulation of DC bus voltage, while retaining the high efficiency and flexibility advantages of droop control. The efficiency of microgrid is improved. The validity and feasibility of the proposed voltage regulation system were verified by simulations and experiments.