wavefront coding is a technology which combination of the optical design and digital image processing. by inserting a phase mask closed to the pupil plane of the optical system, the wavefront of the system is re-modulated. and the depth of focus is extended consequently. in reality the idea is same as the athermalization theory of infrared optical system. in this paper, an uncooled infrared dual field optical system with effective focal as 38mm/19mm, f number as 1.2 of both focal length, operating wavelength varying from 8μm to 12μm was designed. a cubic phase mask was used at the pupil plane to re-modulate the wavefront. then the performance of the infrared system was simulated with codev as the environment temperature varying from -40°c to 60°c. mtf curve of the optical system with phase mask are compared with the outcome before using phase mask. the result show that wavefront coding technology can make the system not sensitive to thermal defocus, and then realize the athermal design of the infrared optical system. © 2017 spie.