A new continuous zoom structure was described, and a new independent zoom group was added on the classical four-group-mechanical-compensation. And larger zoom ratio was achieved by the way of two zoom groups cascading. This mathematical model was deduced. Then, according to the cooled mid-wave infrared detector, a continuous-zoom infrared optical system with large zoom ratio and large relative aperture was designed. And the problem that it’s hard for a zoom optical system to achieve both large zoom ratio and large relative aperture was solved. This optical system can zoom from 6 mm to 330 mm, which means it can reach 55x zoom ratio while the F number is 2 constantly. The working waveband was 3.7~4.8 μm, the cold shield efficiency was 100%.It contains only eight lenses, three of them move in order to change the focal length. The result shows that the zoom curves are quite smooth and the image quality is quite good in the whole zoom range. The laboratory test and the out-door imaging experiment shows that the image quality is quite good in the whole zoom range. It proves the application effect of this new kind of zoom model, and shows that the system reaches its design goal.