Abstract: Athermalization is to eliminate the ambient temperature effect on the performance of optical systems. A passive optical athermalization design was proposed for infrared optical systems. Proceeding from single lens, a set of equations to eliminate the thermal difference of lens group was given. The coefficients for eliminating the thermal difference and chromatism of common infrared materials were described by Cartesian coordinates. The rational combination of infrared materials was derived with the graphical method, and the normalized distribution of focal power was obtained as well. The design process of the passive optical athermalisation was elaborated by an example. The result was analyzed with an optical design software. It shows that the design result meets the requirement of eliminating the thermal difference and chromatism at the temperature range of -40℃～+60℃.