The invention relates to a power grid flow online optimization control method based on a flexible DC transmission technology. The method particularly comprises the following steps: S1, power grid real-time operation information and flexible DC system operation information are regularly acquired from an EMS system; S2, according to the acquired power grid operation mode, the active power of the flexible DC system is adjusted, static safety analysis on the power grid with the flexible DC system is carried out according to an N-1 principle, and an active power interval for the flexible DC system meeting safety constraint conditions of an AC system and the flexible DC system is solved; S3, according to a PQ power circle under different gears, commutation gear change and a modulation ratio constraint condition for the flexible DC system, and the active power optimization interval for the flexible DC system, a reactive power optimization interval for the flexible DC system is obtained; and S4, through the EMS system and the flexible DC system power switching interface, the optimal active and reactive values are issued to the flexible DC system, and field control on the flexible DC power can be realized. Thus, the power grid operation economy and safety can be improved.