The four-level nested neutral point clamped (4 L-NNPC) converter is a new multi-level converter with a simple topology, which is suitable for the medium-voltage application. But it has voltage balancing and Charge-discharge coupling problems under the low-frequency operation. Firstly, the mechanism analysis of NNPC four-level topology floating capacitance fluctuation and the influencing factors are carried out by mathematical methods, and different control strategies are determined according to floating capacitance voltage fluctuation in different operating conditions. Secondly, the paper proposes a new method of adopting different control strategies in the case of small modulation ratios at high and low frequencies, which achieves floating-capacitor voltage fluctuations suppression for the NNPC topology converter in a wide frequency range. Finally, the suppression strategy is verified with simulation on 10 kV four-level nested midpoint-clamped H-bridge inverter, the results show that proposed strategy is correct and feasible.