Three-level converters have been widely used in high-power rail transit applications due to their advantages. However, they also have an inherent problem of neutral-point voltage imbalance. To address this issue in neutral point clamped (NPC) converters, reduce the output phase current distortion, and improve the system operation stability, this paper proposes a new dual degree of freedom neutral-point voltage balance modulation strategy based on the analysis of neutral-point charge. With respect to this modulation strategy, the authors first introduced the causes of the neutral-point voltage imbalance on the DC side of three-level converters from a mathematical perspective, then, analyzed the impact of voltage vector output sequences on the neutral-point voltage in space vector pulse width modulation (SVPWM), and finally improved the traditional voltage vector output sequence according to the above analysis results, and proposed a new dual degree of freedom neutral-point voltage balance modulation strategy based on the characteristics of the improved sequence. This modulation strategy adjusted the action time of balance factors with dual degree of freedom. Theoretically, the proposed new modulation strategy has more controllable charges and better control effect compared with the traditional method. The simulation and test results prove that the proposed new modulation strategy can control the neutral-point voltage faster and more stably. In summary, the proposed new modulation strategy with dual degree of freedom can effectively solve the neutral-point voltage imbalance problem in three-level converters.