For the capacitor voltage balancing of Modular Multilevel Converters (MMC), in order to suppress the submodule (SM) capacitor voltage fluctuation with lower switching frequency, this paper proposes a mark sorting voltage balancing algorithm. The Mark Sorting Voltage Balancing Algorithm (MSVBA) marks SMs in one arm with two different labels according to the set fluctuation threshold, current state and capacitor voltage of each SM, and then sorts and switches SMs respectively. The fluctuation threshold can be adjusted artificially according to various conditions of the system. Two parameters are proposed to evaluate the performance of capacitor voltage balancing algorithms in this paper, which are the average switching frequency of power devices and the voltage fluctuation rate of arm. A detailed model of 21-level MMC-HVDC test system is built in PSCAD/EMTDC, and the MSVBA is compared with two previous algorithms in the test system. The loss distribution of the MMC under different capacitor voltage balancing algorithms is also calculated. The result proves the validity and the feasibility of the proposed MSVBA. Finally, the performances of the MSVBA with different fluctuation threshold are investigated and some suggestions on parameter selection are given. This work is supported by Science and Technology Project of State Grid Corporation of China (No. 52110417001J).