An ultrathin Ti film acting as both an intermediate layer and a dopant was inserted at the interface between Mo and Cu2ZnSnS4 (CZTS) to improve the performance of CZTS solar cells. Different from previously reported blocking layers, the Ti intermediate layer inhibited the formation of a MoS2 layer and voids at the interface between the Mo and CZTS absorber and also influenced the crystallinity, surface evenness, Hall mobility, and absorptivity of the CZTS absorber. When the thickness of the Ti blocking layer increased to 20 nm, the conversion efficiency of the solar cell increased by 57%, along with an increase in the open-circuit voltage of 32%. The effect of the Ti layer on the microstructure and performance of the CZTS film is discussed here in detail. These results serve as guiding principles for preparing high-quality CZTS thin films for potential applications in low-cost solar cells.