Infrared optical systems usually work at a large temperature range due to complex environment. Method of designing three-lens athermalized infrared objective has been introduced. The location of three-lens objective stop was changed to achieve 100% cold stop efficiency. This structure was combined with athermalization model to deduce distribution of the initial focal power. Method of lens materials selection was also given. An example of infrared objective with a 88 mm focal length was designed by this principle. Its wavelength range is 3-5 μm and F number is 2.0. The objective has a perfect image quality. The RMS spot diameter is less than 5 μm, while the MTF reach 0.75@17 lp/mm. The focal length, focal plane and image quality keep steady at temperature range from -20 ℃ to +70 ℃ which shows the feasibility of the design method.