A series of chalcogenide glasses based on Ge-Ga-S-CsI system doped with different Dy3+ ions were synthesized by melt-quenching technique. The refractive indices, Raman spectra, absorption spectra, near- and mid-infrared fluorescence of glass samples were measured. The intensity parameters (Ωi, i=2,4,6), transition probabilities (A), branching ratios (β) and radiation lifetimes (τrad) have been predicted for Dy3+ ions in the samples by using the Judd-Ofelt theory. The mid-infrared fluorescence properties were investigated for different Dy3+ ion concentrations under 810 nm laser excitation. The multiphonon relaxation rates (Wmp) of Dy3+:6H13/2→6H15/2(2.86 μm) and6H11/2→6H13/2(4.38 μm), and the constant related to the e-p coupling strength (α) of Ge-Ga-S-CsI glasses were evaluated.