Abstract: The group velocities of normal modes are widely used in geo-acoustic inversion, and the uncertainty of water depth affects the inversion results. In order to reduce this effect, the law of normal modes' group velocities in Pekeris waveguides versus the water depth and the bottom sound speed is studied in this paper. It is shown that if the frequency is around the Airy frequency of a normal mode, the group velocity of the normal mode is not sensitive to the water depth. But the bottom sound speed has greater influence on the group velocity of that normal mode than the group velocities of lower modes at the same frequency. The group velocities of normal modes are applied to the inversion of the bottom sound speed. The numerical and experimental results show that the influence of the uncertainty of water depth on the inversion results is significantly reduced if the normal modes whose associated Airy frequencies are near the working frequency.