Abstract: The mechanics and prediction methods of duct propeller low-frequency discrete spectrum noise are studied. The acoustic pressure field of duct propeller is split into the propeller radiated field and the duct scattered field based on the linear acoustic principle.Firstly the flow field is analyzed with potential-based panel method to get the unsteady force,and then the pressure data are used as the sources for Ffowcs Williams-Hawings formulation to predict the propeller radiated field,and the duct scattered field is obtained by Kirchoff integral equation.Through the analysis and calculation of duct propeller case,the outcome was obtained that the duct scattered sound field is a small quantity compared with propeller radiated sound field under low-frequency condition and short duct;Also the discrete spectrum noise level of duct propeller is compared with full-scale propeller's discrete spectrum noise level by experiments,conforming the prediction results of duct propeller discrete spectrum noise level are able to reflect the full-scale propeller's discrete spectrum noise level properly.