In order to accurately simulate the external noise of helicopter in over flight， a coupled computational fluid dynamics/computational aeroacoustics（CFD/CAA） model is established. CFD method is used to simulate the aerodynamic force and aerodynamic interference of rotor and tail rotor in aerodynamic calculation. Acoustic infinite element method is used in noise calculation， considering the sound scattering effect of solid boundary and the sound attenuation characteristics in atmospheric propagation. On this basis， taking AC311A helicopter as an example， the calculation and analysis of over flight noise under airworthiness specifications are carried out. Combined with the comparative analysis of flight test results， it is verified that the method established in this paper can be effectively used for the evaluation and analysis of helicopter external noise. Finally， the error analysis is carried out to provide reference for the optimization of external noise evaluation algorithm and simulation model.