To reduce the cost of offshore wind power generation systems, the configuration of the offshore wind farm employing doubly-fed induction generator (DFIG) connected to the diode rectifier unit-based high-voltage direct current (DRU-HVDC) system has emerged as an attractive solution. The control strategy of the DFIG plays a crucial role in ensuring reliable operation of the offshore wind power generation system due to the uncontrollable nature of the diode rectifier unit (DRU). This paper proposes a self-synchronized grid-forming control strategy for the DFIG in offshore wind farm connected to DRU-HVDC system. Considering the unique power characteristics of the DRU, the proposed strategy constructs a novel power synchronization control loop, which achieves self-synchronization of the DFIGs in offshore wind farm without any communication network. Additionally, the harmonic distortion induced by the natural commutation characteristic of the DRU introduces significant electromagnetic ripples to the DFIG through the stator windings. To mitigate this, an electromagnetic oscillation reduction method based on harmonic current injection is incorporated into the structure of the proposed strategy. Simulation results based on MATLAB/Simulink validate the effectiveness of the proposed strategy and the electromagnetic oscillation reduction method.