Abstract: The flexible DC transmission system based on the modular multilevel converter（MMC） has negative damping characteristics in its high-frequency band regularly due to link delay, which is prone to interact with the impedance of the AC system and causes high-frequency oscillation. To completely eliminate the negative damping of MMC, a passive suppression method of parallel C-type filter at AC bus is proposed. By designing the series resonant branch of the C-type filter to resonate at the fundamental frequency, the fundamental-frequency active power loss of the filter can be reduced. In addition, a parameter design method based on the first negative damping frequency band is proposed, and the component cost is considered in the parameter selection. The electromagnetic transient simulation results show that the proposed C-type filter and its parameter design method can significantly reduce fundamental-frequency active power loss, effectively suppress the high-frequency oscillation of MMC, and will not affect the steady-state and transient characteristics of the system.