The present paper has proposed a shell model to describe clad particles of superconductors. The model describes the particles as consist of two parts, i.e. a shell part and a nuclei part. According to the conclusion of the solution of Eliashberg equation in inhomogeneous systems we add the spectrum functions of the two parts together with specific weights. We have calculated the Tc of Pb, Sn, In clad particles with different sizes by the Louie and Cohen's critical temperature Tc fomula which is valid to both weak and strong coupling superconductors. The calcutated results are in agreement with experimental data which have been published so far. In contrast to other authors' calculated results, our results give conclusion which shows that the Tc of Pb clad particles decreases when the size of the particles is reduced.