DC voltage and current of modular multilevel converter（MMC） in voltage source converter based high voltage DC（VSC-HVDC） system cannot be directly control in existing control strategy so as to without full use of the redundant sub-modules. The number of switched on sub-modules per phase N is a constant in existing control strategy. A controllable N strategy is proposed here. At first， on device level a switching algorithm， including number of switch on/off sub-modules， switching time and selection method is proposed so that make N controllable. Then based on the algorithm， AC and DC current hybrid control strategy is implemented on system level with the inner-loop of power/voltage control of the converter. The results of simulation with 71 level MMC illustrate that AC and DC current hybrid control strategy is successful and has good voltage sharing among capacitors in DC link of MMC. It also shows that when DC system voltage fluctuates， the DC voltages fluctuation of sub-modules can be suppressed so as to keep power output smooth.