The conventional flexible DC power transmission based on a modular multilevel converter (MMC) has a problem that a large amount of calculation and a high switching frequency of the sub-module are required for a capacitor voltage equalization strategy of a sub-module during operation. According to this, a capacitor voltage equalization control strategy based on a combination of fast sorting algorithm and improved insertion sorting algorithm is proposed. Reduce the amount of calculation by successively reducing the number of elements to be sorted. Based on this, an improved retention factor insertion method is proposed to further reduce the amount of calculation and switching frequency. Finally, a 31-level MMC model is built in MATLAB / SIMULINK. The simulation results show that the capacitor voltage equalization strategy based on the combination of fast sorting algorithm and improved insertion sorting algorithm can effectively reduce the amount of calculations on the basis of better maintaining capacitor voltage balance.