Abstract: The all-DC offshore wind power system based on series-connected DC ports of wind turbines can effectively reduce the size of offshore platforms and line losses, and it has broad application prospects. However, current research on startup control strategies for offshore wind power systems mostly focuses on traditional AC collection systems, and there's still a gap when it comes to startup control strategies for series-type offshore wind power systems. To address this, a 'staged-step-by-step' startup control strategy suitable for this system is proposed, dividing the full system startup process into three stages: onshore converter station, offshore DC/DC, and DC wind farm. To avoid torque and power shocks during wind turbine startup, and considering the insulation voltage level of the turbine DC ports, the minimum power setpoint is determined based on wind speed differences at the initial startup stage to reduce voltage imbalance at the turbine ports. The effectiveness of the proposed strategy was verified through a full system simulation model built in MATLAB/SIMULINK, ultimately achieving step-by-step startup of the DC wind farm.