Abstract: The offshore wind power DC collection and transmission method has the advantages of avoiding reactive power charging and overvoltage issues caused by AC cables and the like, which will be the main development technique in the future. Different from modular multilevel converter-based high voltage DC (MMC-HVDC), offshore wind power DC collection and transmission system connects the wind power and the onshore MMC by the low and high ports of DC transformer (DCT), the broadband oscillation of which is unclear. So, impedance modeling and oscillation analysis of offshore wind power DC collection and transmission systems are needed. In this paper, the impedance models of system collection and transmission port voltage ports were first established, considering the effects of the onshore MMC on the collection port and the offshore wind power on the transmission port. Then, the impedance small signal coupling pathway which were quantitatively characterized, the dominant influencing factors of the negative damping characteristics were revealed, and the oscillation risks were analyzed. Finally, the electromagnetic transient platform of offshore wind power DC collection and transmission system was built based on Matlab/Simulink, and the simulation experiment verified the accuracy of the ports oscillation analysis, and it was found that the impedance coupling characteristics will affect the oscillation analysis results of the offshore wind power DC collection and transmission system.