The fractional-frequency transmission method has become an important solution for large-scale long-distance offshore wind power collection and transmission due to its advantages in transmission losses and construction costs compared with power frequency and direct current transmission methods.However，the stability research of fractional-frequency offshore wind power systems，especially the analysis under the background of typical large disturbances in system operation，is lacking.A high-order nonlinear state space model of the fractional-frequency offshore wind power system is proposed，and the sector-based nonlinear method is used to deal with the high-order nonlinear part of the model；the Lyapunov energy function of the system is constructed，and the corresponding stable region is calculated.On this basis，based on Lyapunov stability theorem，the interactive stability problem between the permanent magnet direct drive fan and the onshore modular multilevel matrix converter （M3C） under various typical large disturbance conditions is analyzed，and based on the comparative analysis of the stable area，the influence rules of the main circuit and control parameters of the system on the stability under large disturbance are given，which is helpful for system parameter optimization and stability improvement.Finally，the correctness of the Lyapunov energy function and stable region is verified through MATLAB/Simulink simulation examples.