An infrared continuous zoom system is designed. The simple system has a large relative aperture with high transmittance, large continuous zoom ratio, and wide wave band. By introducing a diffractive surface and an aspheric surface, the modulation transfer function (MTF) of the system is above 0.6 for all continuous zoom ratios at the spatial frequency of 14lp/mm. As the system only contains 4 lenses, the transmittance is higher than 80%, and the zoom ratio of the system could also reach 8 continuously. As the color and the off-axis aberration are well corrected, the experiment indicates that the image quality of the continuous zoom system is pretty well.