The offshore wind power is an effective replacement for thermal power plants to realize the high renewable penetrated power system and guarantee the zero emissions. AC transmission and grid connection has the advantage among various solutions from the viewpoint of economics in implementation, where AC submarine cable is the medium of power transmission to deliver the offshore wind farm to the onshore grid. However, such offshore wind power system is more susceptible to the risk of high-frequency oscillation due to the highly distributed capacitance of AC submarine cables. To address this problem, this paper builds the impedance model for offshore wind power via ac submarine cable and further conducts the frequency-domain analysis towards the associated oscillation problem. Firstly, the impedance model of the collection network is constructed and validated. Furthermore, the system stability is analyzed based on the Nyquist criteria so as to analyze the effects of cable parameters on the oscillation frequency of the interconnected system. Finally, the simulation results are provided to verify the correctness of the proposed theoretical analysis.