Using first-principles calculations based on density functional theory with a plane-wave ultrasoft pseudopotential approach, we conduct computations using the CASTEP (Cambridge Sequential Total Energy Package) module within the Materials Studio software. The electronic band structures, densities of states, and optical properties of intrinsic monolayer WTe₂, monolayer WTe₂ with a single tellurium vacancy (V_Te), and rare-earth-doped V_Te-containing monolayer WTe₂ (V_Te-X, where X = Ce, Yb, Eu) are systematically investigated to explore the synergistic effects of rare-earth doping and tellurium vacancy defects on the optical properties of monolayer WTe₂. The results indicate that compared with the V_Te model, the V_Te-X models lead to a more pronounced enhancement of the optical performance in the infrared region (0–1.2 eV). All of V_Te-X structures exhibit metallic characteristics, with a notable increase in the density of states near the Fermi level. In particular, the V_Te-Yb model demonstrates significant improvement in the infrared range: the absorption coefficient, reflectivity, static dielectric constant, and peak value of the imaginary part of the dielectric function are enhanced by factors of 3.76, 1.83, 2.63, and 24.20, respectively, compared with those of pristine monolayer WTe₂. This study provides a theoretical foundation for designing infrared photodetectors based on monolayer WTe₂ substrates.