Disclosed are a collimator (10) and a mounting structure and a mounting method thereof, relating to the field of accessories for X-ray security inspection equipment. The collimator (10) comprises a T-shaped base plate (100) and two lead plates (200). The two lead plates (200) are arranged in parallel and side by side, and are fixed to the vertical plate (103) in the T-shaped base plate (100) and extend towards the transverse plate (104). The collimator (10) is obliquely inserted and mounted in a shielding box (300), with a gap between the two lead plates (200) in the collimator (10) directly facing a first ray opening (3011). The base plate (100) in the collimator (10) is of a T-shaped structure, so as to provide enough mounting space for the two lead plates (200), and the length of the lead plates (200) is shortened to reduce deformation, so as to facilitate the insertion and mounting of the collimator (10), thereby facilitating the disassembly/assembly and adjustment of the collimator (10). Moreover, the collimator can be operated directly by an operator in a standing position, achieving a convenient and labour-saving operation. The security inspection machine using the collimator (10) and the mounting structure thereof is easier to be mounted and adjusted.