With voltage levels and transmission capacity increased dramatically in modular multilevel converter（MMC）based high voltage direct current（MMC-HVDC）projects，the number of sub-modules in each arm increase dramatically，making its control system more complicated.The issues of sub-module capacitor voltage balancing and second harmonic circulation current suppression technology will become more prominent.If the traditional nearest level control（NLC）strategy is used，second harmonic circulation current obviously exists in MMC.In order to suppress second harmonic circulation current，assume that circulation current only contains DC component，DC circulation current formula can be derived based on instantaneous power balance between AC and DC side of the converter.And then each sub-module capacitor voltage reference value in upper and lower arms is deduced.The reference value periodically varies around a certain constant value in a power frequency cycle period，reducing the degree of sub-module capacitor voltage unbalance，and the second harmonic circulation current is validly suppressed.Compared with traditional NLC，the proposed algorithm can reduce the sub-module capacitor voltage fluctuations and improve the output voltage and current waveforms of AC side.When differences exist among the capacitor values，compared with traditional NLC，the proposed algorithm can also reduce the running time of low-capacity capacitors，and prolong their useful life.At last，an eleven-level single end MMC converter simulation model is built to verify the effectiveness of the proposed algorithm.