Tm³⁺-doped chalcogenide glasses in Ge-Ga-S-CsI system were prepared and the properties of glasses including density,absorption spectrum,emission spectra and fluorescence life in the near- and mid-infrared spectral region under 800 nm excitation were measured. The optical parameters such as intensity parameter Ωi (i=2,4,6),predicted spontaneous A,radiative transition rate β and lifetime τrad of Tm³⁺ in Ge-Ga-S-CsI glasses and so on by means of Judd-Ofelt theory according to absorption spectrum were calculated. The fluorescence spectra were investigated with the different Tm³⁺ ion concentration under 800 nm excitation. Also,the emission cross-section at 1.8 μm and 3.8 μm was found using the theory of McCumber and the Futchbauer- Ladenburg equation respectively.