Offshore wind power integration via the fractional frequency transmission system（FFTS）has in recent years drawn attention worldwide.The fractional frequency offshore wind power system consisting of paralleled-connected multi-pole permanent magnet direct-drive synchronous generators（PMSGs），subsea cable，and an onshore cycloconverter possess a number of advantages.In this system，the cycloconverter is used to control the rotating speed of the wind turbines to harvest maximum wind power，and the generator-side converters of PMSGs are no longer needed to reduce the investment and operation and maintenance cost.A quasi-steady-model for fractional frequency offshore wind power system considering the influence of transformers and transmission lines is presented.In addition，a control scheme based on average rotor flux orientation is demonstrated.The multi-machine optimal speed can be obtained via curve-fitting.Furthermore，a simulation model for a fractional frequency offshore wind power system with four PMSG based wind turbines is developed.Simulation results show that the rotating speed of PMSG based wind turbines is able to follow the reference very well under various wind conditions，while the proposed system structure can harvest optimal wind energy.