The existing modal analysis method only judges the harmonic resonance degree according to the amplitude of the modal impedance, which may lead to the wrong analysis conclusion. In view of this limitation, an improved modal analysis method is proposed in this paper, and the half-power bandwidth method is introduced to further solve and improve the modal analysis results. Firstly, the principle of modal analysis method is introduced. Secondly, the resonance analysis is carried out based on the half-power bandwidth method, and the improved modal analysis method is proposed to introduce the performance index to assist in determining the degree of harmonic resonance. Then, construct the multiple grid-connected inverter model and build the node conduction matrix. Finally, the modified modal analysis method is applied to the harmonic resonance analysis, and a simulation is built in MATLAB/Simulink to verify the limitations of the existing modal analysis method and the effectiveness of this method. The results show that the harmonic resonance degree is affected by both the amplitude of the modal impedance and the damping ratio. The improved modal analysis method can be used to evaluate the harmonic resonance degree.