To address the grid stability problem caused by random changes in cable impedance when an offshore wind power generation system is sent out via a power cable, this study proposes a stochastic stability analysis method for an offshore doubly-fed wind power generation system considering the frequency variation of the power cable parameters based on the polynomial approximation theory, which can be analyzed by equating the problem of the random change in the impedance of the offshore wind power public coupling point as a deterministic problem. First, a stochastic dynamics model of doubly-fed wind farm is established by considering variation impedance variation. Second, using the polynomial approximation theory, the wind farm model containing stochastic parameters is approximated for analysis as an equivalent deterministic model. Finally, based on the approximated deterministic model, the grid-connected stability of wind farms with different impedance fluctuation strengths is investigated, and the stable operation interval of the wind farms affected by the impedance is determined. The simulation results show that when the stochastic intensity exceeds a certain threshold, Hopf bifurcation occurs during the voltage and turndown, and the system is destabilized. This study provides a theoretical basis for the stability of offshore wind power.