The development of offshore wind industry demonstrates the capacity expansion of a single wind farm, the increase of the off-shore distance and the standardization of grid integration. These all bring big challenges for the electrical system designs of offshore wind farms. Comparing with traditional power plants, the big offshore wind farm has its unique characters and requirements. This paper describes several existing electric topologies for offshore wind farms, compares the costs and some technical issues of the offshore electric equipment and discusses the problems and prospects of the electrical systems in application for the real offshore wind projects.