The invention relates to a clamping type single sub-module-based starting method for a modular multilevel converter. The starting method comprises the steps as follows: sub-modules in various bridge arms are charged through a starting resistor arranged at a valve side of a transformer respectively in a first pre-charging stage until voltages of capacitors of the sub-modules reach set values; a protective IGBT for unlocking the sub-modules enters a second pre-charging stage; when the voltage between the capacitor C1 and the capacitor C2 in each sub-module reaches a balance state, a preinstalled by-pass knife switch is closed to by-pass the starting resistor; when the voltage of each sub-module reaches a stable value, main IGBTs for unlocking the sub-modules enter an active charging stage; and when the voltage charged to each sub-module reaches a rated working voltage, charging is stopped and starting is ended. Through the clamping type single sub-module-based starting method for the modular multilevel converter, the over-voltage and over-current problems of the sub-modules can be avoided; and smooth starting of the converter is achieved.