Single-atom catalysts (SACs) have attracted extensive attention owing to their high atom efficiency and catalytic activity. However, their superior catalytic properties can hardly be manipulated without a suitable substrate. Here, based on first-principles calculations, we demonstrated that a two-dimensional (2D) defective In2Se3 monolayer with Se vacancy could be used as a proper substrate for various single atoms of transition metals including Zn, Rh, Ru, Pd, Ag, Ir, Pt, and Au. Moreover, the energy barrier of the rate-determining step can be effectively regulated by polarization switching of monolayer In2Se3, which is much lower in M|VSe-In2Se3 (M = Pt, Pd, Ag, and Au) with upward polarization. Especially, Ag|VSe-In2Se3 with upward polarization is identified as a highly efficient catalyst for CO oxidation due to the fairly low energy barrier (0.547 eV).