A long-wave infrared continuous 6× zoom optical system with 640×512 uncooled infrared thermal detector is designed and described. It contains 6 lenses and works at 8~12μm wavelength with the F# of 1~1.1. Based on the theory of zoom optical lenses design, a mechanical compensation structure is determined and the initial structure parameters are calculated. Moreover, the optical system parameters and zoom curves are designed and optimized. The image quality assessment shows that system zooms continuously and smoothly in all 20~120 mm focus lengthens range. The Modulation Transfer Function (MTF) values at 30 lp/mm are all above 0.45 and the spot diagrams’ RMS radius are all less than 6.3 μm which is much smaller than the detector pixel size 17 μm in all focus lengths range. The experiment proves that this zoom system has the merits of high resolution, high zoom ratio, big aperture, big field range, and has steady image plane and zooms smoothly with compact structures and low cost.