Abstract: Modular multilevel converter（MMC） has the characteristics of independently decoupling and regulating active power and reactive power, which makes the MMC flexible DC converter station has the potential to provide reactive power support when AC system voltage sags. This paper proposes a cascaded control strategy of reactive power for MMC flexible DC converter stations,which cascades constant alternating voltage control and constant reactive control. The constant alternating voltage control is added with an arm current auxiliary control, and instantaneous reset control of integration after clearing the fault is also introduced. The Zhangbei ±500 kV four-terminal flexible DC power grid project of China is taken as a simulation example. The results show that compared with the traditional constant alternating voltage control and constant reactive control, the reactive cascaded control mode for flexible DC converter stations can accommodate the steady-state reactive power regulation and transient reactive power support, and output reactive power quickly and smoothly. Furthermore, the designed strategy can ensure the safety of MMC when supporting transient reactive power, and can relieve constant AC overvoltage that may occur at the moment of fault clearing.