Aiming at the characteristics of the three degree of freedom（ 3-DOF） helicopter platform

a frequency domain identification method based on prediction error method（ PEM） is proposed. Mechanism model is constructed

and by using frequency sweep flight technique obtain the input and output data of three channels.Partial coherence function analysis and composite window function analysis are carried out in the system. The identification parameters of state-space equation and the parametric model of helicopter are obtained by prediction error method. It can be verified that the model is accurate and effective through comparison of model prediction response and real flight in time domain.