With the depletion of fossil fuels such as Petroleum, new energy sources have received more attentions. The DFIG wind farm connected to the grid by MMC-HVDC (Modular Multilevel Converter, CMMC) system has become an important means to solve the problem of new energy consumption. The traditional double closed-loop control can not improve the low inertia problem of wind farm and flexible DC system. In order to solve this problem, Firstly, the virtual synchronous generator (VSG) controller of the wind farm side MMC inverter is designed. Secondly, in order to reduce the influence of parameters on VSG and improve its transient and steady state performance, an additional frequency control based on extended state observer (ESO) and terminal sliding mode control (TSMC) is introduced in the VSG frequency controller, and the stability of the controller is proved. Finally, a simulation model of the DFIG connected to the MMC-HVDC is built in Matlab/Simulink. Through simulation analysis, the effectiveness of the Passivity-based Controller to improve the frequency and voltage stability under transient conditions is verified.