The switching devices of three-level NPC inverter are widely used in high-power rectifier due to the advantages of low withstand voltage level and less harmonic content of grid connected voltage. The problem of neutral point voltage balance is always the key of the topology. Through the numerical detection of charge fluctuation of two capacitors at DC side in each switching stage, after formula transformation, the time of positive and negative small vectors of traditional space vector pulse width modulation (SVPWM) is adjusted, so that the charging and discharging time of the two capacitors at the balance point can be kept consistent, and the purpose of neutral point balance control can be achieved. Simulation results show that the method can suppress the fluctuation of neutral point voltage.