Ge-Sb-Se chalcogenide glass is environmentally friendly, and has wide infrared transmitting window, high optical nonlinearity, as well as good mechanical property. These make it a good material for infrared transmission and nonlinear optics. In optical designs, the refractive index ( n) and thermo-optic coefficient ( ζ) of the glass are key technical parameters. In order to predict and tailor the nand ζof Ge-Sb-Se glass, compositions with different chemical and topological features are prepared, their n, ζ, density ( d) and volume expansion coefficient ( β) are measured, and the composition dependence of the parameters is systematically investigated. The chemical feature of the glass is quantified by the percentage deviation of the composition from the stoichiometric ratio and denoted as dSe. The topological feature is represented by the mean coordination number < r> of each atom in the composition. It is shown that the nof Ge-Sb-Se glass increases with dincreasing; the ζdecreases almost linearly with βincreasing; and the βdecreases with dSedecreasing or < r> increasing. When the Ge content is fixed, the dincreases with dSedecreasing or < r> increasing; when the Sb concentration is fixed, the dhas a minimum value at dSe=0. Based on the measured dand n, the molar refractivity ( R _i) of Ge, Sb and Se elements in a spectral range of 2-12 μm are calculated. The obtained value of R _Geis in a range of 10.16-10.50 cm ³/mol, R _Sdin a range of 16.71-17.08 cm ³/mol, and R _Sein a range of 11.15-11.21 cm ³/mol. When the R _iand dare used to compute nof any composition, the discrepancy between the calculated value and the measured one is less than 1%. According to the measured ζand β, the thermal coefficients of the molar refractivity ( φ _i) of Ge, Sb, and Se elements in a wavelength region of 2-12 μm are computed. The optimal value of φ _Geis in a range of 21.1-22.6 ppm/K, φ _Sbin a range of 7.2-8.4 ppm/K, and φ _Sein a range of 90.2-94.2 ppm/K. When the φ _iand βare used to compute ζof any composition, the discrepancy between the calculated value and the measured value is less than 6 ppm/K.