A series of chalcohalide glasses based on the composition 0.9(Ge25Ga5S70)-0.1CsI doped with the different Tm3+/Dy3+ ions ratio were synthesized by melt-quenching technique. The absorption spectra, and mid-infrared fluorescence of different glass samples under 800 nm laser excitation were measured. The results prove that, Tm3+ is an efficient sensitizer, which can enhance the Dy3+: 2.9 μm fluorescence intensity significantly. A decrease in the intensity of 1.8 μm fluorescence and lifetimes of the Tm3+: 3F4 level occurred with increasing the concentration of Dy3+ ions from 0 to 1 Wt% where Tm3+ concentration was fixed to 0.5 Wt%. Also a wide spectral overlap between Tm3+: 1.8 μm emission and the absorption of Dy3+: 6H15/2→6H11/2 showed that the effective energy transfer between the two rare-earth ions was mainly attributed to the resonance energy from Tm3+: 3F4 to Dy3+: 6H11/2 level.