Silicon carbide （SiC） devices feature high switching frequency and low switching loss. Replacing traditional silicon （Si） modular multilevel converters （MMCs） with SiC enables higher-frequency harmonic injection, thereby further reducing submodule voltage fluctuations. By exploring the harmonic injection limits of SiC MMCs, a method for suppressing submodule capacitor voltage fluctuations through coordinated multi-harmonic injection is proposed. Firstly, the frequency boundary for harmonic injection in conventional MMCs is determined based on the traditional MMC model. Secondly, an optimization model for submodule capacitor voltage fluctuation suppression via coordinated multi-harmonic injection is established. By injecting a combination of 2nd, 3rd, 4th, and 8th harmonics, the submodule voltage fluctuations are further mitigated. The results show that the proposed strategy reduces capacitor voltage fluctuations by 71.12% and 54.17% compared with circulating current suppression and typical 2nd harmonic injection schemes, respectively, providing strong support for the lightweight design of MMCs.