The invention discloses a long-wavelength infrared continuous zoom lens. Four lenses are arranged along an optical axis from an object side to an image side in turn, wherein the first lens and the third lens are fixed on the optical axis and are incapable of moving; the second and fourth lenses are capable of moving along the optical axis; the second lens is a zoom lens; and the fourth lens is an offset lens. A system diaphragm is fixed between the second and third lenses. In the invention, the four lenses are used to realize high-magnification zoom, so the system is more compact in structure, lighter in weight, lower in processing cost and simpler in system assembly. In addition, the fixed system diaphragm is arranged between the lenses in the specific location, so the long-wavelength infrared continuous zoom lens can keep a constant relative aperture in a zoom process. Thus, the problem of sensitivity change of the system in the zoom process of the conventional design is solved. Practices prove that the technical scheme has a desirable application effect.