The MWIR continuous zoom optical system has many advantages. A method for designing the system with large zoom range was introduced. Then, a system with 8 lenses was designed. The system works in the wavelength range of 3.7~4.8μm with 10~450mm continuous zoom, and the MTF value in Nyquist limit (16lp/mm) is more than 0.3 over the full range. The F/number of the system is 4. It can satisfy 100% cold shield efficiency.