Abstract: Offshore wind farms connected to the power grid via AC submarine cables may experience harmonic resonance and harmonic amplification due to distributed capacitance of cable lines, which is not commonly observed in onshore wind farms. This poses a serious threaten to the safe and reliable operation of offshore wind farms. To investigate the mechanism of harmonic resonance in offshore wind farms and the influencing factors of harmonic amplification, we first established the state space mathematical model of the transmission network and collector network of offshore wind farms. The calculation methods of harmonic amplification factor, harmonic content, and other indicators were provided. The mechanism and key influencing factors of harmonic resonance were then investigated by means of eigenvalues and participation factors. Additionally, the root locus method was utilized to investigate the impact of grid short-circuit capacity, cable parameters and the number of wind turbine generators (WTGs) on resonance modes. Finally, a model of an offshore wind power system in Jiangsu was constructed on the Matlab/Simulink simulation platform, and the analysis results have confirmed the correctness of the theoretical analysis.