The mechanism of association between modal analysis and harmonic resonance in power system harmonics is analyzed firstly, and the spectrums of matrix eigenvalues and critical modal admittance in series harmonic resonance are drawn according to the loop impedance matrix. Then, the system topology corrected with dummy branch is proposed based on the characteristics of series resonance and harmonic excitations to analyze the harmonic resonance of power network. The result of analysis is compared with the result of system admittance spectrum scan to verify feasibility and accuracy of the harmonic resonance frequency calculation method. Next, defining the sensitivity matrix, the sensitivity of key model eigenvalue to each network element electrical parameters is derived using fundamental loop matrix to determine relevance between system components and loop impedance matrix elements. The comparability among the sensitivities of branch elements is improved using the standardization. Finally, simulation analysis of the IEEE30 bus system is made, and the impact of series resonance on the element sensitivity, the series resonance positioning and transmission range are obtained.