Abstract: For the fact that the capacity scale of some AC power grid at the sending-end of HVDC transmission systems is small, the proportion of wind power is relatively high, and frequency control ability is poor, it is of practical significance for the HVDC system to control the frequency of the sending-end power grid. In this paper, we proposed a frequency control method of hybrid cascaded HVDC transmission system connected to isolated island wind farm to sending-end grid, which makes the hybrid HVDC system and wind farm jointly support the frequency of the sending-end power grid. Aiming at the problem that the hybrid cascaded DC system reduces the transmission DC current due to the participation of wind farm in frequency control when the modular multilevel converter connected to the power grid operates at full load, we proposed an improved frequency control of hybrid cascaded DC system under the heavy load operation of converter to improve the frequency regulation ability under this condition. Finally, the corresponding simulation model was established on PSCAD/EMTDC simulation platform to verify the effectiveness of the proposed frequency control method.