Abstract: The voltage source converter(VSC) based DC technology is crucial for improving the rate of the penetration of renewable energy resources and achieving carbon peaking and carbon neutrality. Modeling with low order and moderate accuracy is of great significance for fast analysis of DC voltage dynamics and enhancement of system stability. Therefore, the reduced-order model of a VSC-DC grid is built to characterize the dynamics of DC voltage. First, the DC-side impedance of the VSC station is built in the form of second-order expression, which turns the droop-based outer control loop into a specified admittance with resistance-inductance characteristic. The VSC stations with different control are equivalent to a unified model of series-parallel impedance. Next, the reduced-order transfer function matrix of a VSC-DC system is formed based on the grid topology. The interactions of multi-VSC stations on DC voltage dynamics through DC grid coupling are investigated quantitatively by frequency response analysis, thereby guiding the parameters design of the droop gain and the outer control loop. Finally, the correctness and effectiveness of the proposed reduced-order model are validated by simulation.