Cu2BaSnS4 (CBTS) is a low cost, non-toxic material composed of abundant elements with a large optical absorption coefficient. However, in CBTS solar cells, elemental Ba reacts readily with water, which decreases the quality of the CBTS film. Therefore, in this paper, we present an easy and cost-effective approach to prevent the hydrolysis in CBTS films by adding an ultrathin Sn mask layer at the surface of the CBT precursor film. Adding a Sn mask layer can improve morphology, reduce recombination, and enhance the quality of p-n junction between CBTS and the CdS film by inhibiting the reaction between Ba and water. When the thickness of Sn mask layer was increased to 5 nm, the open circuit voltage, short current density, fill factor all increased and the conversion efficiency of the solar cell increased from 0.27% to 1.21%. However, the second phase SnS2 appeared at the surface of the CBTS film when the thickness of the Sn mask layer increased from 5 to 15 nm, which may severely impair the p-n junction resulting in deterioration of the performance of the CBTS device.