This paper studies the voltage control principle of flexible DC transmission power grid.Firstly,the main 3 voltage control strategies and their characteristics are reviewed.Then the voltage control strategy for DC grids in which primary voltage regulation and secondary voltage regulation coordinate with each other is proposed.The proposed primary voltage regulation is based on the law of voltage droop control with voltage dead band whose realization method and logic diagram have been described in the paper,while the proposed secondary voltage regulation is based on the control criterion of keeping the voltage of the benchmark voltage node constant.After introducing the concept of the benchmark voltage node,the paper describes the benchmark voltage node selection method and the implementation of the secondary voltage control.A 4 terminal DC grid is constructed as the test system.Based on the test system,the design procedure for the voltage droop control with voltage dead band is described in detail.Two operation situations are tested on the test system.One is when the load of the DC grid changes and the other is when the benchmark voltage converter drops out.The results show that the proposed DC voltage control strategy has good control performance and can be used in general DC grids.