Abstract: Optical system with a large relative aperture has a larger entrance pupil than its counterparts when their focal lengths are equal,so more energy collected and further distance detected. But due to its large aperture, the aberrations increase, which requires more lenses to correct and consequently cause application restrictions by volume and weight. To tackle this issue, zoom parameters were calculated and optimized reasonably, moreover by adopting compounded aspheric and diffractive surfaces, a reimaging zoom midwave infrade (MWIR) system with only 8 lenses was designed. The F number is 1.8 and the spectrum is 3 m~5 m. For a 320240 pixels detector with cooled focal plane array（FPA）, the magnification is 5,and the four fields of view (FOVs) are 30 mm/60 mm/90 mm/150 m. The optical aperture stop matches the detector-s and the cold stop efficiency is equal to 100%. Two reflective mirrors are applied to fold the optical path, giving a compact volume of around 210 mm100 mm85 mm. Less lenses and simple zoom machinery effectively reduce the weight.