The invention provides a random switching frequency modulation method for a three-level NPC converter. The method comprises the steps that 1 according to a carrier pulse width modulation method, a three-phase modulation wave is acquired; a spatial vector pulse width modulation policy is used; a fixed switching period is set; a, b and c three-phase switch states and the respective action time thereof are solved and output; 2 according to the limit value of a ripple current effective value, a random switching period which meets the ripple current limit value is solved; and 3 the random switchingperiod in the step 2 is used to replace the fixed switching period in the step 1, and the respective action time of three-phase switches under the random switching period is calculated to form the final modulation waveform. According to the invention, the modulation method adjusts the switching period according to the predicted ripple current effective value, so that the ripple current effectivevalue meets the limit value of the ripple current effective value under the random switching period; the harmonic content of the output current of the converter is effectively reduced; and the methodhas a good practical value.