The electromechanical transient characteristics of a power system with fractional frequency wind power system（FFWPS）are analyzed.Firstly，a quasi-steady state model of the cycloconverter is established，which describes the relationship of voltages，active power and reactive power between the two sides of the cycloconverter during the transient process.Secondly，the calculation method is proposed to analyze the transient stability of the power system with FFWPS.This method combines the quasi-state equations of the cycloconverter and the network equations on both sides.Then these simultaneous equations are solved with the alternating iterative method.Thirdly，the transient stability of the power with FFWPS is analyzed，the results show that the damping of the rotating speed of a faulted wind generator in the fractional frequency system is bigger than that in the industrial frequency system.Moreover，when a fault occurs at the low frequency side，the cycloconverter can effectively restrain the oscillation of the generators at the industrial frequency system.