The invention provides a paper cup machining apparatus and specifically provides a fully automatic paper cup forming machine. The fully automatic paper cup forming machine comprises a machine body, a cup body forming mechanism disposed on the machine body, a transfer rotating disc linked to the cup body forming mechanism, a cup bottom conveying mechanism, a cup body and cup bottom combining mechanism, a heating mechanism, a bottom folding mechanism, a knurling mechanism and a crimping assembly, wherein the cup bottom conveying mechanism, the cup body and cup bottom combining mechanism, the heating mechanism, the bottom folding mechanism, the knurling mechanism and the crimping assembly are arranged along the transfer rotating disc successively in order. A cup body steering mechanism is arranged between the cup body forming mechanism and the transfer rotating disc. The cup body steering mechanism comprises an adjusting rotating disc and at least two cup body holding sleeves disposed on the adjusting rotating disc and capable of turning on the adjusting rotating disc. The adjusting rotating disc is driven to rotate by a driving component. The adjusting rotating disc is provided with a turning component driving the cup body holding sleeves to turn. The fully automatic paper cup forming machine increases the machining efficiency, is high in cup body position adjusting speed and is simple in structure.