The invention relates to an automatic assembling machine for a shaft gear. The automatic assembling machine comprises a rack and a distribution control box, wherein a gear mounting turntable is arranged on the rack and connected to a gear mounting motor; placing holes in shaft fit are uniformly formed in the gear mounting turntable; gear mounting carriers concentric with the placing holes are arranged on the placing holes; and a gear loading device, a product loading device, a gear tightening device and a product unloading device which are matched with the placing holes or the gear mounting carriers are sequentially arranged on the outer side of the gear mounting turntable, and the product loading device comprises a product conveyor trough and a product pick and place device matched with the product conveyor trough. As the gear mounting turntable is arranged, the placing holes in shaft fit are uniformly formed in the gear mounting turntable, the gear mounting carriers are arranged on the placing holes, and the gear loading device, the product loading device, the gear tightening device and the product unloading device are sequentially arranged on the outer side of the gear mounting turntable, automatic continuous operation for gear installation is realized, and the gear mounting efficiency is greatly improved.