In order to effectively improve the frame of mid-wavelength infrared measuring system, based on a 320×256 cooled staring focal plane array(FPA) detector, pixel size 30μm×30μm, a special optical system combined with time-sharing exposure technology was designed. In this paper, the working wavelength is 3μm∼5μm, the temperature range is -30°C∼+40°C, this system can realize 200mm focal length, the F-number is 4 and the full field of view is 3.44°. A re-imaging refractive system was adopted in this designed optical system consists of main optics and projection components. First of all, the working principle of the measuring system was introduced in detail. Secondly, on the basis of variety of the distance and temperature, a focusing lens was presented in this system to adjust to produce a clear image. Last but not the least, to improve image quality and environment adaptability, the analysis of temperature change and narcissus effect was described particularly. The design results prove that at the spatial frequency of 17 lp/mm, the MTF of the optical system is greater than 0.58(the axis MTF of the optical is greater than 0.65), the system can offer a high resolution and excellent images. The measuring system implements 400fps measuring system consisting of four 100fps detectors. Copyright © 2018 SPIE.