The invention relates to a collimation mechanism for a convexity meter, and belongs to the field of nuclear technology application. The collimation mechanism includes two sets of collimation devices with the same structure, and each set of collimation devices includes 3-stage collimators, and the collimators from the first stage to the third stage are arranged sequentially along the radiation direction of a radiation source. The first-level collimator includes a first tungsten plate; the second-level collimator includes second to fifth tungsten plates, two mounting plates, a bottom plate, a flat plate, and a fixing block, and the two mounting plates are placed on the flat plate, The two mounting plates are respectively connected with the second to the fifth tungsten plates, the two mounting plates are connected with the flat plate, the two mounting plates are connected, the flat plate is connected with the bottom plate, and a spring is pressed between the flat plate and the fixed block; the third level collimation The device includes the sixth tungsten plate, and the sixth tungsten plate is fixed by the top wire; after this three-level collimation, the ray beam emitted by the ray source becomes a collimated ray beam, which solves the problem that the convexity instrument lacks the collimation function for the ray beam emitted by the ray source Problems, improve the accuracy of product detection.