Cu2ZnSnS4 (CZTS) compound is a promising candidate for thin-film solar cells since its constituents are earth abundant and nontoxic. One of the major challenges to obtain a high-quality CZTS absorber is to overcome the interfacial mismatch and formation of secondary phases between the CZTS and Mo substrate during the sulfurization process. Generally, the CZTS decomposed into Cu2S, ZnS, and SnS phases during sulfurization, and high-density voids and cracks were observed. These micro- or macroreactions changed the stoichiometry of CZTS. In this paper, we present the insertion of a SnS buffer layer at Mo/CZTS interface to inhibit the undesired reaction and improve the thin-film quality. The insertion of the thin SnS buffer layer prevented the CZTS absorber to contact directly with Mo and suppressed the present of secondary phases, pores and cracks. Crack-free and smooth morphology was obtained. The cell efficiency was significantly improved.