In this paper, the narrow-angle parabolic equation (NAPE) model applied on atmospheric ducts is solved by using the improved discrete mixed Fourier transform (IDMFT) method which is an efficient form of the split-step Fourier transform (SSFT) algorithm under the impedance boundary conditions. In order to verify the accuracy of the narrow-angle parabolic equation method, the numerical results are compared with the experimental data in the literature as well as the results of mode theory and the Advanced Refractive Effects Prediction System (AREPS) software. Finally, the effects of the receiving antenna height and the evaporation duct height on the propagation loss in the evaporation duct are analyzed in details.