The offshore wind power gradually develops towards the direction of far-reaching sea and large capacity, and the offshore wind power DC transmission system based on diode rectifier unit has a great development prospect, and its main circuit parameter design is an important part of the HVDC engineering design. Combined with the basic principle of wind power system and converter station, the working characteristics of offshore wind power DC transmission system based on diode rectifier unit are analyzed. For the important equipment, such as converter transformer, smoothing reactor and AC filter, the calculation formulas of main circuit parameters are derived, and the complete design idea and calculation process of main circuit parameters are given. Lastly, the parameters designed are verified by PSCAD/EMTDC simulation which can provide reference for the future design of offshore wind power DC transmission system based on diode rectifier unit.