The electronic and optical properties of the defect chalcopyrite XGa2S4 (X=Zn, Cd, Hg) compounds are studied based on the first-principle calculations. Its structural properties are consistent with the earlier experimental and theoretical results, and its electronic and optical properties are discussed in detail in this paper. The results indicate that the three compounds described hare exhibit an anisotropic behaviour in the intermediate energy range (4 eV10 eV), and an isotropic behaviour in the low(4 eV) or high(10 eV) energy range. The refractive index curves of ZnGa2S4 and HgGa2S4 have an inflection point at the plasma frequency p, and their reflectivity reaches a maximal value at p and then declines sharply. Moreover, the calculated optical properties indicate that these compounds can serve as shielding and detecting devices for ultraviolet radiation.