The relationship between athermalizing infrared optical system and zoom lens is established. The process of designing a athermalizing infrared optical system could be regarded as a process of generalized zooming. The process of designing a generalized zoom lens is described in detail, and the correlative principles are deduced. This process consists of four steps, which are setting zoom positions in the system that satisfies image performance demand at normal temperature, entering private glass catalog at different temperatures, establishing relationship between different zoom positions and analyzing thermal effect, then adding factor that could eliminate thermal effect.