An optical coupled-dipole model was proposed in order to study the coupling between the probe tip and the rough sample in SNOM. In this model both the optical probe tip and the sample protrusions were represented by optical polarizable dipole spheres. The induced polarization effects on the sample surface can be replaced by the image dipoles in the context of quasi-static electromagnetic field approximation. Applying the radiation theory of the dipole,a set of self-consistent equations have been established to describe the field distribution at the sites of the probe tip and the sample protrusions. This method permits us to analyze the physical mechanisms of the interaction between the probe tip and the rough sample surface in SNOM intuitively. On the basis of this approach,the characteristic of the near-field imaging and the localized optical resonance of the metallic sample were discussed. The calculating result indicates that its near-field images were directly related to the incident light frequency,......