The invention discloses an active frequency control method and system for offshore wind farm flexible-direct current grid connection, and the method comprises the steps: obtaining a frequency signal of a shore AC power grid access point, and transmitting the frequency signal to a control protection unit of a flexible-direct current system shore converter station; when it is detected that an inter-station communication system of the flexible-direct current system shore converter station and an offshore wind power plant control and protection system is normal, sending the frequency signal to a frequency active modulation module of the offshore wind power plant control and protection system through the inter-station communication system; and transmitting the active power regulation quantity generated by the frequency active modulation module to an offshore wind plant energy management system based on the frequency signal, wherein the offshore wind plant energy management system modulatesthe active power output by the offshore wind plant according to the active power adjustment quantity. Therefore, the active power output by the offshore wind power plant is modulated, and the shore alternating current system is supported for frequency modulation. The frequency active adjustment function of the offshore wind power to the shore AC power grid access point through flexible-direct access is realized.