Abstract: For medium wave cool 640512 detector with staring focal plane array, a MWIR continuous zoom optical system with large zoom range was designed. The optical system imaged thrice. The operating wavelength range was 3.7-4.8 m, F number was 4 and the zoom ratio was 30:1. The optical system could realize 23-701 mm continuous zoom and meet 100% cold shield efficiency with smooth zoom locus. Three infrared materials of Si, Ge and ZnSe were used in the optical system. The system could correct the off axis aberration and higher order aberration by leading the aspheric surface and the diffractive surface. The MTF was above 0.15 within the whole focal range at the spatial frequency of 30 lp/mm and it approached the diffraction limit. The results show that the MWIR continuous zoom optical system has large zoom ratio, high resolution, small volume and excellent image quality.