Abstract: For the 320240 pixels uncooled infrared focal plane array(FPA) detector, a midinfrared zoom optical system was designed working at 3.7 m~4.8 m waveband.It contains 6 sphere lenses. All lens elements are made up of silicon and germanium materials which can transmit 3.7 m~4.8 m infrared rays. The F number is 2.5, the back working length is 20 mm, and it can realize 15 mm~150 mm continuous zoom. Designed results show that the modulation transfer function (MTF) value of zoom lens is greater than 0.6 at 16 lp/mm in the full field of view (FOV), and no less than 0.7 in the 0.7 FOV,which means near the diffraction limit generally. The uncooled infrared unit for plane arrays can catch more than 70% infrared rays energy. The lens system is characterized by small F number, long back work length and all spherical surfaces. And the image quality meets the requirements within -20 ℃~60 ℃.