The mechanical and thermal properties of (100-x)Ge20Sb10Se65Te5-xCsI(x = 0, 1, 2, 3, 4) glass was studied. It is found that with increase of CsI, the short-wave cut-off wavelength of infrared transmittance moves from 1000 nm to 900 nm, and the long-wave cut-off wavelength reaches 20 mu m. The glass transition temperature decreases from 531 K to 521 K. The crystallization peak temperature T-p decreases from 709 K to 670 K. The Vicker's hardness decreases from 142H(v) to 134H(v). The fracture toughness increases from o.48 to 0.57. The specific heat capacities were measured by the isothermal step method and it shows that the specific heat for the sample with CsI is larger than that without CsI. For Ge20Sb10Se65Te5-4CsI glass, the transmittance changes from 70 % to 53 % if the treatment temperature is between 280 degrees C and 310 degrees C. When it is kept at 310 degrees C for 40 h, the Vicker's hardness and fracture toughness of the glass can reach 160.56 kgf.mm(-2) and 0.82 kgf.mm(-1/2), respectively.