The invention discloses a ripple current peak value based random switching frequency modulation method for an NPC converter. The method includes steps of 1, obtaining a three-phase modulation wave according to a carrier wave pulse width modulation method, setting a fixed switching period and solving and outputting a, b and c phase switch states and corresponding acting time of the three phase switches; 2, according to the maximal value and an inflection point value of the slope of ripple current in the three phases, determining the ripple current maximal value, and solving a random switching period meeting a ripple current limit value; 3, using a random switching period for replacing the fixed switching period, and calculating three-phase switch acting time in the random switching period and forming the finally modulation waveform. According to the invention, the ripple current maximal value is obtained through comparison and the switching period is adjusted, so that the ripple currentmaximal value meets the limit value of the ripple current valid value in the random switching period. Therefore, the harmonic content of current output by the converter is reduced effectively and a good practical value is achieved.