For the neutral point potential balancing problem of three level Neutral Point Clamped (NPC) inverter, this paper analyzes the AC unbalance of neutral point voltage based on carrier pulse width modulation, and proposes an accurate compensation method to reduce the AC ripple of neutral point voltage. The method eliminates the AC unbalance of the neutral point voltage by injecting an optimum compensation value to the phase voltage duty cycle, and the accurate mathematical calculation of the optimum compensation value is given. According to the calculation results, the limitation of completely eliminating the AC ripple of neutral point voltage and the applicable range of the neutral point AC ripple voltage reduction method are analyzed. A three level neutral point clamped inverter model is built using Matlab/Simulink. The proposed method is simulated and the simulation results show its correctness and effectiveness. This work is supported by National Key Research and Development Program of China (No. 2017YFB0903100) and National High-tech R & D Program of China (863 Program) (No. 2015AA050104).