In this paper, the authors have theoretically probed into the inherent relation between the wavelength of Rayleigh wave and the surveying depth, computed the curve of ellipticity of particle motion of Rayleigh wave (equal to the ratio of the horizontal and vertical vibration value) with the depth corresponding to the unit wavelength. Finally,the authors have dealt in detail with the conversion coefficient problem of wavelength and depth based on a large number of computations of forward models.