Abstract: Waveguide characterization estimation in shallow water using the complex mode approxi metion of acoustic field is presented. The advantages of the method are that (l) the dependence on the environment parameters will obviously decrease. (2) the waveguide characterization parameters(k_m, β_m)can be obtained the same time, when the fine structure in the bottom medium, such as the number and thickness of layers, the velocity of compressive and shear wave, and attenuation is less known or unknown. (3) it can extract the bottom reflection coefficient V(k). Numerical examples illustrate that (1) the accuracy of k_m is at least 10^-5, the related error of β_m is mostly less than 20%, and the smaller the mode number, the smaller the related error is; (2) the reflection coefficient in the the bottom at low grazing angle can be obtained using by a vertical array which the total number of samples is N=18 and length L=90 m. The experimental data of 500 Hz is emphatically discussed here. The results for f=500 Hz wide impulse acoustic signal source which locates at z_s=11 m, r_s=21 km show that the extracted accuracy of k_m is between 10^-4 and 10^-5, the related error of β_m is between 9% and 26%.