The volume and cost of a modular multilevel converter (MMC) are mainly limited by the sub-module capacitor. Active injection of double-frequency circulating current or third-order harmonic voltage is an effective method of capacitance reduction. However, most of the existing harmonic injection strategies apply offline table lookup methods to calculate injection parameters. When the working conditions change, the injection parameters need to be recalculated, which are not suitable for scenarios with frequently changing operating conditions. In addition, there is still lack of analysis on the coupling effect of coordinated injection of the two methods. Firstly, the influence of coordinated second- and third-order harmonic injections on the voltage fluctuation of the sub-module capacitor is analyzed, and then the effect on the MMC operating characteristics is studied and the constraints of the injection parameters are given. Secondly, based on the voltage fluctuation of the sub-module capacitor, the optimization results of the coordinated harmonic injection parameters are given. Finally, the simulation model of MMC is built in PSCAD, and the proposed strategy is simulated and verified. The results show that the coordinated harmonic injection strategy can effectively suppress the capacitor voltage fluctuation of the sub-module.