Due to the severe offshore environments， the maintenance of offshore wind turbine is very difficult and the failure rate is very high. To improve the condition monitoring technologies will be beneficial to promote the performance of offshore wind turbine and the productivity of offshore wind farm. Some recent novel theories and methods about the condition monitoring technologies of offshore wind turbine are reviewed and classified， including monitoring system development， condition information collection， condition information transmission， fault diagnosis， state control， operation cost analysis， and so on. Outstanding issues and development trends are also summarized. The results of this work will contribute to the support for constructing the high-performance condition monitoring and control system for offshore wind farm.