On the condition of the underwater acoustic signal, the perturbed amplitude would be small, and the general hydrodynamic equation may be linearied. On the basis of this approximation, the horizontal dispersion relation and the form of the water surface are obtained. The results show that the water surface transversal mini-wave has the characteristics of shorter wavelength and lower velocity, therfore it can modulate the light indensity of the incident laser beam on the water surface， which lays the theoretical foundation for the detection of underwater signal.