On the basis of cooled staring focal plane array (FPA) detectors with 640×512 pixel and 20 μm pix. el elements, a mid-infrared continuous zoom optical system with 30 zoom ratio is designed. Operation waveband of the system is 3.7~4.8 μm, F number is 3.7 and focal length range is from 11.64 to 349.2 mm. The system includes two parts such as a zoom system and a re-imaging system, which includes 12 lenses. Main function of the re-imag. ing system is to realize 100% cooled diaphragm efficiency and reduce radial size of optical systems. Space resolu. tion of the system can be 21 lp/mm and the energy concentration rate is above 75% in 25 μm. And imaging quality of the system is much better. Optical passive athermal design is realized in the range of -40~+60 ℃.