The invention relates to a mechanism for sleeving a bolt with a nut and a sleeving method. A carrying plate is movably connected to the upper portion of a base, and a pressing plate is detachably installed above the carrying plate and can be positioned and compressed on the carrying plate; the base is provided with three groove holes allowing a screw of the bolt to penetrate; a plurality of limiting holes are formed in the carrying plate, and positioning blocks capable of positioning the nut are installed in the limiting holes; and the pressing plate is provided with a plurality of round groove holes allowing the head of the bolt to penetrate, and each round groove hole corresponds to one limiting hole. In the sleeving process, the pressing plate is opened, and the nut is placed into the limiting holes; the pressing plate is positioned and compressed on the carrying plate, the bolt penetrates the round groove holes in the pressing plate and is screwed into the nut, and the screw of the bolt penetrates the groove holes in the base; and the pressing plate is lifted up, the carrying plate is rotated by a handle, the bolt and the nut which are sleeved well are separated from the carrying plate, and finished products are taken out. The mechanism and the sleeving method solve the problem that when the bolt is sleeved with single or multiple nuts manually, the efficiency is low, the mechanism and the sleeving method are suitable for polygonal nut sleeving, and sleeving and fixing of depth nuts can be achieved.