The invention relates to a modular multi-level inverter sub-module capacitor voltage balance control method and belongs to the technical fields of voltage source inverters and direct current transmission. The method includes the steps that each sub-module control pulse delay control quantity by tracking the average value of sub-module capacitor voltage of an arm as a target, the rising delay time and falling delay time of the control pulse of the sub-module are controlled according to the direct current voltage adjusting direction and the arm current direction, and the average direct current electric current of a sub-module capacitor within a period is changed to achieve balance control of the sub-module capacitor voltage. The modular multi-level inverter sub-module capacitor voltage balance control method does not need complex sorting calculation, thereby greatly reducing calculation amount. The method can be used for a control unit distributed in each sub-module. The complexity of the implementation of the method is not increased with the increasing of the number of arm sub-module cascading. The method is especially suitable for a modular multi-level inverter of great level number.