CZTSSe-based solar cells with a structure of Al/ITO/ZnO/CdS/CZTSSe/p-WSe2/Mo (denoted as WSe2-CZTSSe solar cell) were prepared on the Mo-coated soda-lime glass (SLG) with a pre-sputtering metal tungsten (W) layer. Comparing with the conventional CZTSSe solar cell with a structure of Al/ITO/ZnO/CdS/CZTSSe/n-MoSe2/Mo (denoted as CZTSSe solar cell), the WSe2-CZTSSe solar cell shows a significant improvement in open-circuit voltage (V-OC), short-circuit current density (J(SC)) and fill factor (FF), and so its power conversion efficiency (PCE) enhances. The largest enhancement of the PCE is from 4.13% of the CZTSSe solar cell to 5.45% of the WSe2-CZTSSe solar cell. The increased V-OC and J(SC) are mainly due to the enhancement of photogenerated current density (J(L)) and decrease of reversion saturation current density (J(0)), while the increased FF is ascribed to the increase of shunt resistance (R-Sh). The influence mechanism of the WSe2 buffer layer on J(L), J(0) and R-Sh is also investigated in detail. Our results present a route of increment of PCE by reducing and even removing back contact barrier between CZTSSe and MoSe2 layers.