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 n and ζ of 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 n of Ge-Sb-Se glass increases with d increasing; the ζ decreases almost linearly with β increasing; and the β decreases with dSe decreasing or <r> increasing. When the Ge content is fixed, the d increases with dSe decreasing or <r> increasing; when the Sb concentration is fixed, the d has a minimum value at dSe=0. Based on the measured d and 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_Ge is in a range of 10.16-10.50 cm³/mol, R_Sd in a range of 16.71-17.08 cm³/mol, and R_Se in a range of 11.15-11.21 cm³/mol. When the R_i and d are used to compute n of any composition, the discrepancy between the calculated value and the measured one is less than 1%. According to the measured ζ and 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_Ge is in a range of 21.1-22.6 ppm/K, φ_Sb in a range of 7.2-8.4 ppm/K, and φ_Se in a range of 90.2-94.2 ppm/K. When the φ_i and β are used to compute ζ of any composition, the discrepancy between the calculated value and the measured value is less than 6 ppm/K.