Abstract: Reducing the cost of offshore wind power sending system is key to promote the development of offshore wind power sources. The technical solution based on transmission by diode rectifier possesses good economy. However there are defects in the system such as the wind farm cannot be black-started and the transmission system cannot provide AC voltage for the grid-connection of wind farm. To explore more feasible approaches based on the above mentioned technical solution, a design of improved diode rectifier based transmission system was proposed. On the basis of configuring auxiliary AC line, by adding reactive power compensator at the offshore end or making the inverter on mainland side to directly participate in system coordination, the black-start and the steady state power flow direction control could be realized. The theoretical analysis, system scheme design and control strategy design for the proposed technical scheme were carried out. In the simulation environment of PSCAD/EMTDC, a model of offshore wind power transmission system was constructed. Both methods were verified by simulation results and were proven to satisfy the reliable transmission of offshore wind power.