Abstract A series of chalcohalide glasses based on the composition of 0.9(Ge 30Ga 5Se 65)-0.1CsI with different Tm 3+/Dy 3+-codoped ions concentrations were synthesized by melt-quenching technique. The absorption spectra and 2.9μm mid-infrared fluorescence spectra of glass samples under 800 nm laser excitation were measured. The results showed that Tm 3+ was an efficient sensitizer, which could enhance the Dy 3+:2.9 μm fluorescence intensity significantly. The effective energy transfer between the two rare-earth ions were mainly attributed to the resonance energy transfer from Tm 3+: 3F 4 to Dy 3+: 6H 11/2 level. Emission cross section of 2.9 μm mid-infrared luminescence was also investigated according to Judd-Ofelt theory, σ e =2.51×10 −20 cm 2.