Unexpected sideband harmonics of the cascaded H-bridge (CHB) inverters will appear in unbalanced conditions among cells and may result in harmonic pollution and even harmonic resonance. The linear controller to suppress the output voltage distortion is destined to be ineffective due to the nonlinear characteristic of sideband harmonics. Focusing on this problem, in this article, we propose a closed-loop harmonic suppression method by adjusting the phase-shift angles for the N-cells CHB inverter. The harmonics suppression mechanism is revealed from a vector synthesis point of view. No complicated trigonometric function calculations and optimization algorithms are required. This method can achieve the suppression of the harmonic distortion around any multiplicative switching frequency. Phase-shift angles are adjusted continuously in real-time. The superior harmonic performance achieved by the proposed method is verified by the simulation and experimental results.