Long control link delays can lead to negative damping in the impedance range of flexible DC transmission systems based on modular multi platform converters (MMC). These can easily interact with distributed capacitors in the AC system and cause high-frequency oscillations. Although existing active damping suppression strategies can effectively suppress high-frequency oscillations, they will deteriorate the impedance characteristics of the mid frequency range and increase the risk of mid frequency oscillations. To effectively suppress mid-to-high frequency oscillations, this paper proposes a passive damping suppression strategy by paralleling a passive impedance reshaping device at the AC side bus of the MMC. First, an MMC simplified model is derived and its accuracy is verified. Secondly, impedance reshaping device parameters are designed based on the Nyquist stability criterion and minimum power loss constraint to achieve impedance reshaping of the MMC system. Finally, the impedance method is applied to compare the impedance characteristics of the MMC system before and after reshaping, and the correction effect of the impedance reshaping device on the impedance of the MMC system is analyzed. An MMC electromagnetic transient simulation model with a passive impedance reshaping device is built in PSCAD/EMTDC. The results show that this suppression strategy can effectively suppress the oscillation phenomenon of the MMC system in the mid-to-high frequency range, with little impact on the steady-state characteristics of the system, and low fundamental frequency power loss.