The theoretical model based on the Fresnel diffraction integral is developed to describe the surface thermal lens (STL) signal excited by a cw modulated top-hat beam. The three-dimensional profile of temperature rise, the radial profile of the surface displacement and the diffraction signal of the probe beam obtained with both top-hat and Gaussian beam excitations are compared by numerical calculation. The dependence of the surface thermal lens amplitude on the experimental parameters are discussed. Numerical results show that the top-hat beam STL instrument is more sensitive than the Gaussian beam STL instrument, with a maximum sensitivity enhancement factor of approximately 2.