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 standard parabolic equation can estimate propagation angle from 0 degree to 20 degree, and second-order parabolic equation method removes phase inaccuracy of 20° to 43°. Parabolic equation solution is much cheaper than solving the elliptic equation both in memory and running time. Meanwhile, calculation efficiency of standard parabolic equation and second-order parabolic equation method are almost the same. From 0° to 43° of sound propagation angle, the error between 2nd order parabolic equation prediction and analytical solution is less than 5%.