In order to meet the requirements of the miniaturization of the new large-format high-resolution uncooled infrared detector（1024×768 pixels）and the infrared thermal imaging system, the continuous zoom structure with mechanical positive group compensation is adopted. An uncooled infrared continuous zoom imaging optical system with focal length of 25-75 mm is designed through the reasonable distribution of optical focal degree of each lens. The operating band is 8-12 μm, and the F-number is 1.2.The system consists of five lenses with a total mass of only 255 g and optical length of 125 mm. The design result shows that the system is small，simple and compact, and good in imaging. The modulation transfer function value is greater than 0.3 at the corresponding characteristic frequency（36 lp/mm）of the detector, which can meet the application requirements.