With the development of infrared surveillance technology, the short focus zoom LWIR lens has been paid to more and more attention. In this paper, dual fields zoom lens with the fields of 20 degrees x25 degrees/8 degrees x10 degrees are designed by two different methods. The main specifications of the lens include: aperture is 1 / 2, detector resolution is 640 x 512, pixel size is 15 mu m, temperature adaptability is -40 degrees to + 60 degrees. And we compare the volume, weight, imaging transmission, transmissivity and economy of the lens by different methods. It is found that the re-imaging method has smaller volume, weight and economy. While another imaging method has better transmittance, so the imaging system can reach higher noise equivalent power at the same image quality.