In diode rectifier unit (DRU) based DC transmission systems for integrating offshore wind farms, the DRU lacks controllability and hence grid-forming control of wind turbine generators is required for power balance and voltage control of the offshore electrical system. This paper first establishes linearized state-space models of both the grid-forming PMSG wind turbines using P-U/Q- control and the DRU-MMC hybrid DC transmission system. The model’s accuracy is validated through step responses in time domain simulation. Subsequently, a discrete domain state-space model of the whole system is derived. Using a system of three offshore wind farms connected via a DRU-based DC transmission system as an example, a full-system small-signal model is constructed with a discrete state-space approach. Through eigenvalue analysis, the oscillatory modes of the system and their stability are investigated. The analysis results show that there exist wide frequency band eigenmodes attributed to circuit parameters, machine side converter control of PMSG, or interactions between the grid side control of PMSG and DRU, or between the MMC control and AC system. Especially, super- and sub-synchronous frequency as well as low frequency oscillation modes exist due to control interactions among wind farms, whose stability is sensitive to parameters of control designs and may result in unstable oscillation. This research provides theoretical support and analysis methods to understand the dynamic behavior of offshore wind farms connected via DRU-based DC transmission systems, providing guidance for grid planning and operation with offshore wind power integration.