The invention discloses a mould-less large-caliber PVC pipe dynamic molding device and method. The device is mainly composed of an extruder, a film bracket, bracket rolls, a bracket motor, a heating box, a three-point circle setting instrument, rollers, pressure rollers, supporting rollers, a pipe support frame, a support frame motor, guide rollers, a guide roller motor, guide rails and an ultrasonic welding instrument, etc. The three-point circle setting instrument can be used for continuous molding, and the PVC pipe length is not restricted by a roller mould. Compared with existing PE large-caliber pipes, the PVC raw material is much cheaper than PE, thus greatly lowering the production cost. The device provided by the invention omits the large-caliber roller mould, thus being able to save demoulding time and improving the molding efficiency. Also, the ultrasonic welding temperature is precisely controlled, thus effectively avoiding the PVC degradation problem caused by a high melting welding temperature. And the welding is fast. The mould-less large-caliber PVC pipe dynamic molding method and device provided by the invention can be used for molding large-caliber PVC smooth wall round pipes and outer corrugated pipes simultaneously.