Cu2BaSn(S1-x Sex)4 has shown great prospects in photoelectric field due to earth-abundance, low toxicity, cost efficiency, direct bandgap, high absorption coefficient (>104 cm-1) and reduced anti-site disorder relative to Cu2ZnSn(S1-xSex)4. Fully-tunable ratio of S/Se is the key to broaden the bandgap of Cu2BaSn(S1-xSex)4. Here, we introduce a thionothiolic acid metathesis process to readily tune stoichiometry of Cu2BaSn(S1-xSex)4 films for the first time. Different stoichiometric Se/(S+Se) of Cu2BaSn(S1-xSex)4 from 0 to 1 can vary the bandgap range from 2 to 1.68 eV. The grain size of Cu2BaSn(S1-xSex)4 films can be grown more than 10 μm. The optimized bandgap and high-quality growth of Cu2BaSn(S1-xSex)4 films ensure the best power conversion efficiency of 2.01% for solution-processed Cu2BaSn(S1-xSex)4 solar cells. This method provides an alternative solution-processed way for synthesis of fully stoichiometric Cu2BaSn(S1-xSex)4.