The invention discloses a method and device for correcting modulation degrees of modularized multi-level converters. The method comprises the following steps of: obtaining a switch function and arm current of a converter; iteratively calculating arm output voltage of the converter according to the switch function and the arm current, and in the iterative calculation process, correcting the arm current and the switch function on the basis of the arm output voltage obtained in each iterative calculation so as to ensure that deviations between an amplitude and a phase angle of a fundamental component of the arm output voltage and an amplitude and a phase angle of a fundamental modulation voltage of the converter are located in a corresponding preset ranges; and calculating a modulation degreeof the converter according to the amplitude of the fundamental component of the arm output voltage obtained via the iterative calculation. The method and device are capable of solving the problem that the converter modulation degrees solved by adoption of basic operation principle of voltage source converters have relatively large deviations with practical modulation degrees in the prior art.