Because of the randomness，intermittence and fluctuation of wind resource，its reliability has great influence on the security of large － scale integrated power system． How to accurately evaluate the reliability of wind power generation system poses a new challenge． Firstly，the structure and output of wind power generation system are analyzed． A method based on equal expected energy not supplied ( EENS) is proposed to substitute a generator with the same capacity for wind turbine " string" equivalently． In addition，the reliability state is divided by the component state characteristics． At last，the system reliability is comprehensively evaluated by the time，the output and the system． According to the simulation on an actual wind power system，the result shows that the index system can accurately evaluates the reliability of wind power generation system．