Offshore wind power plants (WPPs) using AC submarine cables connected to the power grid may experience harmonic resonance and harmonic amplification that are not common in onshore wind farms under the action of the distributed capacitance of submarine cables, which seriously threatens the safe and reliable operation of offshore WPPs. To investigate the influencing factors of harmonic amplification in offshore WPPs, a scalable state space-based modeling method for large-scale offshore WPPs were established. Moreover, the root locus method was used to study how the factors including that grid short-circuit capacities, cable parameters and the number of wind turbine generators (WTGs) affect the resonance modes. Finally, an offshore WPP in Jiangsu was built on the MATLAB/Simulink simulation platform, and the analysis results verify the correctness of the theoretical analysis.