Abstract: The impedance characteristics of the converter and the connected AC line are obtained by impedance frequency scanning. Then, the impedance models of the converter and AC line are established by mathematical fitting method. The amplitude-frequency and phase-frequency characteristics of the converter and the connected AC line are calculated by MATLAB. The Nyquist criterion is used to determine the range of high frequency oscillation generated by the flexible DC. On this basis, a new method based on LCL filter is proposed to suppress MMC HVDC high frequency oscillation. The PSCAD simulation model is established in Chongqing to Hubei MMC HVDC BTB project, and the correctness of the proposed method is verified by electromagnetic transient simulation in detail.