Abstract: A geometrical acoustic method based on image-source method and physical acoustic method is developed to calculate the target echo in the shallow water waveguide.The incident rays are reflected by two boundaries for many times,and then the back rays become countless.The total back scattering field is obtained through summing up the scattering field produced by each combination of incident rays and back rays.The echo of the 10 m-radius pressure release sphere in Pekeris waveguide with the range is calculated by the geometrical acoustic method.Compared with the results calculated by the wave acoustic method in the obtained literature,it shows that both are in accordance on average value and descend trend.The following results indicate that the difference between Equivalent Target Strength (ETS) in shallow water and the Target Strength (TS) in free space for spheres and certain other rounded objects is small. However,the ETS of some targets such as cone-shaped is quite different from TS in free space,which can lead to large errors in estimating a target's scattering property using traditional sonar equation.Compared with the method of wave acoustics,the geometrical acoustic method not only has the definite physical meaning but also can calculate the echo of complex objects in shallow water waveguide.