Abstract: In recent years, flexible DC grid attracts a lot of attentions and develops fast, whose stability is the important premise to make the hybrid AC/DC power system operate normally. Aiming at the flexible DC grid, the linearized model of voltage source converter（VSC） station and the flexible DC grid with different control methods is established, and the interaction model of flexible DC grid is established, which takes oscillation mode of DC voltage in the flexible DC grid as analysis target. Based on this, a virtual damping analysis method with more clear physical meaning is proposed, and its concept and principle are introduced. Then,virtual damping index which can evaluate the influence of VSC station control on the stability of flexible DC grid is proposed, and the impact of VSC on the oscillation mode of DC voltage with different control methods is quantitatively analyzed. Furthermore,based on the virtual damping index, a coordinated optimization method for the control parameters of VSC stations in flexible DC grid is proposed, which improves the small-signal stability of the DC grid. Finally, taking a three-terminal flexible DC grid as an example, the calculation method of virtual damping in VSC stations is presented in detail. And the effectiveness of parameter optimization method in multiple VSC stations and the advantages of the method compared with the traditional parameter optimization method is verified in the complex ten-terminal flexible DC grid with droop control.