The imbalance of midpoint potential directly affects the output voltage waveform quality of the diode-clamped three-level inverter (NPC) and the service life of the device. Aiming at the problem that the traditional virtual space vector method cannot actively control the midpoint potential offset and the switching frequency is higher, an improved virtual space vector method is proposed to reduce the switching frequency under the low modulation regime, When the modulation regime is low, two combinations of small vectors of equal amplitude and opposite sign to the total effect on the midpoint potential are obtained by using only positive small vectors for one small vector and only negative small vectors for the other small vector. Then dynamically select the combination of small vectors to be used by judging the size of the midpoint potential in real time, so as to realize real-time control of the midpoint potential offset. The method can ensure the balance of the midpoint potential as well as quickly control the midpoint potential offset, and also greatly reduce the switching frequency due to the non-use of redundant small vectors. Finally, a simulation model is built in Matlab/Simulink, and a simulation comparison between the traditional virtual space vector method and the improved virtual space vector method proposed in this paper is carried out to validate the method in the low-tuning regime.