Based on the analysis of the space vector modulation strategies for three-level inverters, the duration assignment of the two redundant switching states and the selection of different switching sequences can be considered as two degrees of freedom. The impacts of these two degrees of freedom on the quality of output current are evaluated by using the mathematical model of current ripple established in this study. Moreover, according to the principle of minimum rms value of output current ripple, the optimal assignment of the two redundant states and the optimal distribution of switching sequences are determined. Thus, the optimisation of the output current waveform is realised. The simulation and experimental results show that, for the proposed strategy, the total harmonic distortion (THD) of output current and the weighted THD of output voltage are both lower than that of the conventional strategies.