Lead-free double perovskite, CsAgBiBr, with higher stability and lower toxicity than those of its lead counterparts, has been considered a promising alternative for next-generation photovoltaic materials. For practical applications, a facile deposition method that could be used to fabricate high-quality double perovskite films with large grain size is highly desired. However, such kind of facile method has never been established for CsAgBiBr. Herein, high-quality CsAgBiBr thin films with an average grain size of approximately 0.5 μm were successfully deposited via a simple one-step spin-coating method by using dimethyl sulfoxide (DMSO)-N,N-dimethylformamide (DMF) mixture with optimized volume ratio as the solvent and chlorobenzene (CB) as the antisolvent. On the basis of satisfactory quality of the film, efficient (>1%) CsAgBiBr perovskite solar cells were constructed. Furthermore, the photo-generated charge-carrier transfer from CsAgBiBr to the adjacent carrier extraction layers was systematically investigated via femtosecond transient spectroscopies. This study offers a new pathway to acquiring high-quality CsAgBiBr thin films and provides a useful guide toward the development of high-efficiency double perovskite solar cells in the future.