As offshore wind power moves into deeper waters, the required capacity of offshore wind farms continues to increase, and the demands for stability in the power system rise accordingly. This paper proposes a grid-forming control strategy for a fully DC transmission offshore wind series aggregation system. The strategy enables wind turbine units to establish and support voltage. The paper establishes a system of the offshore wind power DC series aggregation, and conducts detailed analyses of the wind turbine series aggregation grid-formation mode and the DC -DC topology of wind turbines. A PFM-PSM control strategy utilizing a DC/DC topology has been adopted, and an output voltage control strategy for the grid-forming series aggregation system is proposed. The model was also simulated using the RT-LAB real-time simulation system, and the simulation results verified the feasibility of the proposed strategies under various operating conditions.