VSC-HVDC technology has the advantages of fast decoupling and independent control of active power and reactive power, and it has certain reactive power support to AC system. Firstly, by analyzing the basic control strategies of VSC-HVDC system, the paper finds the characteristics and mechanism of dynamic reactive power support when using different control strategies. Then, this paper also analyzes how the dynamic reactive power is supported to AC system when the near-zone fault and the far-zone fault occur, with different active power being transmitted by the VSC-HVDC line. Finally, this paper proposes different reactive power control strategies when VSC-HVDC system runs in different scenarios.