In order to study the coupling between the probe tip and the rough sample in scanning near-field optical microscopy(SNOM),a quasi-static electromagnetic field approach is adopted and a theoretical model for SNOM is proposed.In the present model both the optical probe tip and the sample protrusions are represented by polarizable dipole spheres.The induced polarization effects on the sample surface can be replaced by the image dipoles.Applying the radiation theory of the dipole,a set of equations to describe the field distribution at the sites of the probe tip and the sample protrusions have been established.This set of field equations can be solved self-consistently.The results are completely the same as those obtained by means of the dyadic electromagnetic propagator formalism.The principal advantage of this approach lies in its practicality and simplicity in calculation.