The invention discloses a full-automatic rotary tail shearer for PET bottle preforms. The full-automatic rotary tail shearer for PET bottle preforms comprises a conveyor belt, a group of guide blocks,a strengthening device and a tail shearing device. The group of guide blocks are fixedly mounted on the conveyor belt; the strengthening device is mounted above the conveyor belt through the fixed bracket; the tail shearing device is mounted above the conveyor belt through a lifting bracket; a group of strengthening rollers are symmetrically mounted in the strengthening device; a group of rotating blades are symmetrically mounted in the tail shearing device, and a group of guide rails are arranged right below the group of rotating blades. In this way, the tails of the PET bottle preforms aresheared automatically, a purpose of primarily forming a product can be achieved, secondary manual shearing and processing are avoided, the labor is saved, the labor intensity of the worker is reduced,the production cost is lowered, and the production efficiency is improved.