Abstract: In order to solve the problem that a certain offshore wind power is connected to the power grid through modular multilevel converter（MMC）,and broadband oscillation occurs between the onshore converter station and the power grid,the frequency domain model of MMC control system in dq coordinates is established based on the idea of impedance analysis and MMC control system structure. The d and q coupling parts of the MMC control system model are converted to the time domain and abc coordinate system for simplification,and the system decoupling is realized. Then the frequency domain model of the MMC control system in abc coordinate system is established,and its equivalent impedance model is established based on the analysis of the MMC equivalent circuit. The characteristics of converter impedance model and power grid impedance model are analyzed by impedance analysis method,and the mechanism of oscillation is verified. Finally,the measures to suppress broadband oscillation are analyzed,and the solution of additional band stop filter is given,which is verified by the oscillation phenomenon of field cases.