Abstract: Based on density functional theory, the effects of applied electric field and biaxial strain on the electronic structure of monolayer Sb₂Te₂Se were systematically studied. The results show that the tensile strain can make the conduction band near the Fermi level steeper and the valence band flatter. The tensile strain can reduce the band gap value of monolayer Sb₂Te₂Se. The effect of compressive strain is opposite to that of tensile strain. The biaxal tensile strain can enhance the density of states(DOS). The increase in state density will improve the thermoelectric performance of monolayer Sb₂Te₂Se. When the electric field strength is in the range of 0.1-0.3 V/Å, the conduction band and the valence band near the Fermi level are insensitive to the external electric field, and the band gap hardly changes with the external electric field, which is mainly due to the shielding effect.