Solvent engineering is an important route for improving the quality of solution-processed copper-zinc-tin-sulfur-selenium (CZTSSe) thin films. Here, blended solvents composed of dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) were used to prepare CZTSSe precursor solution. The addition of DMF improved the wettability of precursor solution on Mo-coated substrates and modified the CZTSSe formation. Impacts of different DMF contents in solvents on morphological, structural, optical and photovoltaic properties of CZTSSe films were fully studied. Compared with the pure DMSO-based precursor solution, incorporation of an appropriate amount of DMF into DMSO can effectively increase the grain size and improve the film compactness, while adding excess DFM decreased the crystallinity and deteriorated the device performance. By optimizing DMF content, the carrier lifetime of CZTSSe thin film was increased from 2.78 ns to 5.07 ns and the energy conversion efficiencies of solar cells were improved from 4.77% to 6.03%.