The invention relates to the field of automatic assembly machines, and discloses an automatic assembly device of a captive nut, which comprises an assembly frame platform, a stamping die, a feeding device and a nut-feeding mechanism, wherein the assembly frame platform realizes fixation and movement of the whole device, the stamping device is positioned above the assembly frame platform, the pressing die comprises a pre-bending punch, a bending slanted sliding block, an upper die assembly jacking the punch, and a lower die assembly comprising a bottom plate, the feeding mechanism comprise a feeding mechanism fixing device driven by a first cylinder, a feeding mechanism movable device driven by a second cylinder, and a third cylinder in a horizontal direction, the third cylinder is connected with the feeding mechanism movable device, a nut-feeding mechanism comprises a fifth cylinder, a square push rod and a nut sliding chute, the push rod is driven to move in the nut sliding chute, and an output of the nut sliding nut extends into the stamping die. The invention realizes automatic assembly of the captive nut, has capabilities of effectively saving labor cost and increasing production efficiency, and is suitable for production of massive captive nuts.