Abstract: To improve the efficiency of bypass duct sound propagation and liner optimization, a technique for calculating the effect of aft fan sound propagation was studied. Based on the theory of parabolic approximation, theoretical equation of 2nd order parabolic equation is developed. The accuracy of standard parabolic equation and 2nd order parabolic equation was 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 2nd parabolic equation removes phase inaccuracy of 20 degree to 43 degree. 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 2nd parabolic equation are almost the same. From 0 degree to 43 degree of sound propagation angle, the error between 2nd order parabolic equation prediction and analytical solution is less than 5%.