Abstract: By use of modular multilevel converter(MMC), the independent control and rapid regulation of reactive power can be implemented, flexible MMC-HVDC converter station can be used as a plant-level (or station-level) adjustment resource to promote the optimization level of regional power system reactive power. Therefore, a research on reactive power regulation capability of flexible MMC-HVDC converter station was performed. Firstly, according to the topology of typical single-terminal flexible MMC-HVDC converter station a mathematical model was proposed. Secondly, two major categories of influence factors, i.e., the equipment capability constraint influencing the reactive power regulation capability and the operating state of the system, were analyzed, and three constraint conditions, i.e., the rated capacity of converter transformer, the overcurrent withstand level at the valve side of converter transformer and the voltage modulation ratio, were put out, besides, the calculation steps of reactive power regulation capability was derived. Finally, taking Zhangbei ±500 kV four-terminal flexible HVDC project for example, its reactive power regulation capability under such steady state operating conditions as different active loads, DC voltage and voltage at grid-connected point and corresponding main constraint conditions were expounded, thereby, the obtained results provide the basis for the design of reactive voltage control strategy of flexible MMC-HVDC converter station as well as for the reactive power security margin control.