The invention relates to an automatic assembling machine for a piston pin. A power distribution control box is arranged on a rack, a working platform is arranged on the power distribution control box, a fixing device and a bolt mechanism which are in fit with each other are arranged on the working platform, the fixing device comprises a fixed seat on the working platform, and a fixing plate in fit with a piston is arranged on the fixed seat; the bolt mechanism comprises a mounting base arranged at the right side of the fixed seat, a loading block is arranged on the mounting base, a loading chute in fit with the piston pin is formed in the loading block, the loading chute is in locational fit with a bolt hole of the piston, a pushing device in fit with the loading chute is arranged at the right side of the loading chute, and the pushing device is connected with the power distribution control box. The automatic assembling machine for the piston pin has the advantages that the piston is fixed by virtue of the fixing device, the piston pin in the loading chute is pushed into the piston pin hole by virtue of the bolt mechanism in fit with the fixing device, bolting efficiency is greatly improved, bolting time is shortened, and bolting strength also can be accurately controlled, so that inserted depth of the piston pin is appropriate, and multiple piston pins are the same in the inserted depth.