Developing highly efficient and economical bifunctional catalysts for overall water splitting is challenging but a great concern for the renewable energy conversion and applied catalysis towards a sustainable development. Herein, by employing first principles calculations, the catalytic performance of various transition metal (TM) atoms supported on single-layer ternary Ga2FeS4 for water splitting has been systematically investigated. Strikingly, it was evidenced that the sandwich Ga2FeS4 can form a tunable interfacial polarization field under external strain, which can effectively regulate its catalytic performance. Specifically, Au-Ga2FeS4 under a 0.5% tensile strain presents good bifunctional catalytic performance, with the low overpotentials of 0.03 and 0.48 eV, respectively, for the HER and OER, outperforming the experimentally identified Pt and IrO2. Considering the massive possibilities of these ternary substrates and the easily accessible characteristic of external strain, the findings highlighted here can provide a useful theoretical guideline for designing efficient bifunctional SACs in overall water splitting.