The invention relates to an offshore wind power DC (Direct Current) transmission system connected with a distributed offshore platform in series, and belongs to the field of offshore wind power transmission and grid connection. The offshore wind power DC transmission system comprises multiple offshore wind turbine groups, multiple offshore current conversion platforms, seabed DC cables and an ashore inverter, wherein an AC (Alternating Current) gathering bus of each offshore wind turbine group is connected to an AC input end of the corresponding offshore current conversion platform; DC ends ofthe multiple offshore current conversion platforms are sequentially connected in series and then are connected to a DC end of the ashore inverter through the seabed DC cables; an AC end of the ashoreinverter is accessed to an onshore AC power grid. In the offshore wind power DC transmission system disclosed by the invention, all distributed offshore current conversion platforms are smaller in volume, and the construction difficulty is lower; power conversion of AC/DC is realized by adopting a unidirectional current type modular multi-level current converter, and the offshore current conversion platforms are lower in cost and higher in compactness.