Abstract: To improve the efficiency of bypass duct sound propagation and liner optimization, a technique for calculating the effect of aft fan sound propagation is studied. Based on the theory of parabolic approximation, theoretical equation of second-order order parabolic equation method is developed. The accuracy of standard parabolic equation and second-order order parabolic equation method is analyzed and compared, and then followed with validation cases for parabolic equation prediction result and analytical solution. Validation case result indicates that compared with the traditional parabolic equation method, the second-order parabolic equation method can effectively improve the accuracy of high-order modes and large angle sound propagation, increasing the sound propagation angle limit from 20 ° to 43 °. The calculation efficiency of the second-order parabolic equation method is almost the same as that of the traditional parabolic equation method. From 0° to 43° of sound propagation angle, the error between second-order parabolic equation prediction and analytical solution is less than 5%.