In order to ensure the mutual inertial support and frequency response between the wind farm system and the power system, an additional frequency control strategy for converter station level is proposed. The additional control strategy adds the f-Udc droop characteristic in the WFVSC constant-frequency control and the GSVSC constant- DC voltage control, respectively, the relationship of the frequency in the AC power systems can be flexibly supported by each other under the condition of no need of communication, thus easing the burden of frequency modulation in the accident system. Finally, the model of wind farm and DC transmission is set up in PSCAD / EMTDC. The simulation results show that the proposed additional frequency control strategy enhances the inertial support level and the frequency control reserve of the system.