Athermal design is necessary to design a LWIR optical system for working under temperature range of -40-+60 ℃, because the infrared optical materials could transform extremely with the change of temperature. On the basis of introduction of athermal system and analysis on the effects of optical elements, an infrared telephoto objective lens was designed in the way of optical passive compensating, which working waveband was 8-12 μm long wave infrared band, F# was 2, the field of view was 6.8°,uncooled focal plane array was adopted. A prism was added in the system to achieve athermalization and compact structure without adding a special surface, the telephoto ratio of this system was 0.69. All of the surfaces were designed to be standard sphere, which was of benefit to processing, alignment and testing. The result of design indicates that the optical transfer functions are close to the diffraction limit in the required temperature range and the design requirement is met.