This paper proposed an improved carrier phase disposition modulation strategy for modular multilevel converter (MMC), namely a single-carrier modulation strategy. It uses a single triangular carrier to achieve multilevel modulation. It has advantages of both good harmonic characteristics for carrier phase disposition modulation and switch balanced distribution of carrier phase shifted modulation. In order to solve the problem of submodule capacitor voltage imbalance of the MMC, this paper presents a switching allocation strategy according to submodule capacitor voltage sorting. Simulation results show that the proposed modulation strategy and switch balanced distribution strategy were verified by the PSIM simulation.