Abstract The quality of Cu2ZnSnS4 (CZTS) absorber layer, Mo/CZTS interface and heterojunction CZTS/CdS are the most important factors directly affecting the performance of CZTS solar cells. On the base of high quality absorber layer and back contact interface, the effects of post-heat treatment (PHT) process of CZTS/CdS on the quality of heterojunction and device performance have been comprehensively investigated. The PHT process can promote the elements in the heterojunction region interdiffuse between absorber and buffer layer, i.e., more Cd elements diffuse to CZTS, and Zn, Cu elements diffuse to CdS. Consequently, the elements interdiffusion affects the chemical composition of the absorber layer surface and the band alignment of the heterojunction. After applying the PHT process, the surface of CZTS becomes Cu-poorer in comparison to the CZTS bulk. Due to the diffusion of Cd2+into the CZTS layer, the free carrier density is increased. In addition, PHT process can also promote the recrystallization of CdS and thus greatly improve the crystallization quality of CdS. Owing to the more favorable band alignment at the CZTS/CdS interface as well as better crystallinity of CdS, higher quality heterojunction was obtained. Finally, at the optimized PHT temperature of 275 °C, a pure CZTS solar cell with active area efficiency of 9.75% (total area efficiency: 8.90%) has been fabricated. Based on the application of PHT process, reducing the deep level defects in CZTS bulk is expected to further improve the device performance.