The invention discloses an ultra-thin infrared collimator based on a fly-eye lens, and belongs to the technical field of infrared optics. A fly-eye lens assembly, a target lens assembly and a light source system are sequentially mounted in a box body from front to back, a front cover plate is fixedly mounted at the front end of the box body, an infrared output window is formed in the middle part of the front cover plate, and a rear cover plate is fixedly mounted at the rear end of the box body; the fly-eye lens of the fly-eye lens assembly is formed by combining a plurality of small lens unitsin the form of a rectangular array, a target image array is engraved on the target lens in the target lens assembly, a target image array surface is positioned on one focal length focal plane of thesurface of the fly-eye lens array, a plurality of target images 'F' engraved on the target image array surface are in one-to-one correspondence with the image surfaces of the small lens units on the fly-eye lens array, and targets are consistent; and the focal points of the small lens units in the fly-eye lens array coincide with the centers of the target images 'F' in the target lens image array.The ultra-thin infrared collimator is used for detecting and identifying infrared optical targets.