Abstract: Since infrared systems have features such as good atmospheric adaptability, passive features, antijamming ability and small volume, light weight and little power consumption, they have found wide applications in navigation, surveillance, guidance and military camouflage target detection. In recent years, the demand for infrared zoom systems is increasing in proportion with the development of infrared technology and its applications. The design of a zoom lens for middle infrared system with a staring focal plane array detector is described in this paper. The operating wavelength of the system is 3～5 μm, and zoom ratio is 20∶1. The image quality achieved with the zoom lens is evaluated with the CODE V optical design software.