Limited by the aberration of marginal rays in the aperture,it is difficult to accommodate large aperture,dual field-of-view,image rotation,and athermalization in a compact refractive infrared lens. Under further constraints of a given large entrance-pupil distance,an overall length,and more,Gaussian optics and athermalization theory are used to determine the initial structural data of a lens,and a structural isoperformance method is used to improve the design and optimization process. To enhance the compactness of a lens,compound measures are applied. The measures are incorporating lateral switch-over of FOV,axial compression of telephoto objective,aspherical surfaces for high-order aberration correction and material combination along with active focusing for athermalization. A long-wave infrared afocal lens is designed,which is 160 mm in entrance pupil diameter in NFOV and only 345 mm in length. In a range of -40 ℃~+60 ℃,the lens performs near diffraction-limited MTFs and encircled energies,and the distortions is less than 3 %,which satisfy the requirements of imaging capabilities in a FLIR and distant target detection and angle-measurement capabilities in an IRST. The practicability of the design method is verified.