The invention relates to a constant aperture zoom lens. The zoom lens comprises a first lens group (G1) having positive focal power, a second lens group (G2) having negative focal power, a diaphragm (STOP), a third lens group (G3) having the positive focal power and a fourth lens group (G4) having the positive focal power which are successively arranged from an object side to an image side along an optical axis. During a zooming process of the zoom lens from a wide-angle end to a telescopic end, the second lens group (G2) moves from the object side to the image side along the optical axis while the fourth lens group (G4) also moves along the optical axis so as to correct changes of an image plane position during the zooming process. A maximum aperture of the zoom lens is less than or equalto 1.4, a maximum field angle of the wide-angle end is above 75 degree, the field angle of the telescopic end is less than 25 degree, a focal length zoom ratio is greater than 3.0, and the shortest optical total length is less than 90mm so that a visible and infrared light confocal state can be realized. Temperature compensation is considered, and in a temperature range of -40 DEG C to 80 DEG C,the same resolution power with that at a normal temperature can be guaranteed without refocusing.