Abstract: Double perovskite materials (A₂BB^'X₆) have emerged as promising candidates to replace lead-based perovskites due to their environmentally friendly nature, structural stability, and tunable optoelectronic properties. In this study, 14 structurally stable K₂NaB^'X₆ (B^' = Sc, Y, Al, Co, Sb, Ga, In; X = F, Cl, Br) double perovskites were screened based on the octahedral factor (μ) and tolerance factor (t). Their electronic band structures, density of states, optical properties, carrier effective masses, and exciton binding energies were systematically investigated using first-principles density functional theory (DFT). Results show that the six materials, including K₂NaCoCl₆, possess direct bandgaps ranging from 1.05 to 2.97 eV, making them suitable for use as light-absorption layers. Further introduction of vacancy defects (VK, VNa, VB^', VX) revealed that Cl vacancies (VCl) are the most easily formed and can induce metallization in the materials. K and Na vacancies enhance light absorption in the visible region, while Co vacancies reduce the exciton binding energy, facilitating exciton dissociation. This study elucidates the regulatory mechanism of vacancy defects on the optoelectronic properties of double perovskite materials, providing a theoretical basis for the design of lead-free perovskite optoelectronic devices.