The equation is obtained for calculation and modeling crystallization surface  Y₂O₂S  in quasiternary system Y₂O₂S–Eu₂O₂S–Ga₂S₃ in the temperature range 1200–2400 K. The equation includes analytical expressions of liquidus curves of Y₂O₂S in quasibinary boundary systems Y₂O₂S–Ga₂S₃, Y₂O₂S–Eu₂O₂S and function defined by a limited number of experimental data of DTA for the ternary system. This equation permitted to model the surface of crystallization Y₂O₂S as a kind of dependence of liquidus temperature on the composition using the program grafikus.ru/plot3d .