The invention relates to a method for positioning a conjugate focus of an infrared Cassegrain collimator of a group of concentric spherical mirrors, and the method is characterized by comprising the following steps: (1) alignment is carried out to a fixed spherical mirror and a calibration spherical mirror on an device, thereby allowing two working surfaces of the fixed spherical mirror and the calibration spherical mirror to be coaxial and concentric; (2) the group of the concentric spherical mirrors after the alignment are arranged in an infrared light path, the positions of an infrared detector and an infrared laser are adjusted till the infrared detector and the infrared laser are coaxial and conjugate and concentric zero stripes are generated on the infrared detector; (3) the calibration spherical mirror is removed, the infrared Cassegrain collimator is arranged at the position in the infrared light path, the position of the infrared Cassegrain collimator is adjusted till an interference image which is the same as the step (2) is generated on the infrared detector; the position of the infrared detector is the position of the conjugate focus of the infrared Cassegrain collimator.