The invention provides a method and device for modulating a converter. The method for modulating the converter comprises the steps that a first voltage quantity is obtained according to a three-phase alternating voltage, a three-phase alternating current, a direct voltage and a direct voltage set value of the converter; power quality parameters of the converter and system loss parameters of a system where the converter is located are obtained; a self-adaptation coefficient used for modulating the converter is obtained according to the power quality parameters and the system loss parameters; a second voltage quantity is obtained according to the self-adaptation coefficient and the first voltage quantity, and the converter is modulated through the second voltage quantity. By means of the method and device, switching loss in the converter can be adjusted flexibly, and the flexibility and practicability of adjustment of the minimum switching loss are improved while the running performance of the converter is taken into consideration; besides, both power quality and system loss are considered to modulate the converter, so the overall generating capacity can be increased, and huge theoretical value and construction value are obtained.