A system for in-line controlling of the ultrasonic welding of plastic components of an electronic cigarette, in which plastic components are welded to one another at a welding zone to define a welded group; the plastic components are such that the welded group has an air passage duct having a first open end and a second open end. The system includes: a chamber for receiving the welded group; a closure for closing the first open end; air supply for supplying air into the chamber. The system further includes an air exit, connecting the welding zone with the outside of the chamber; a continuous pressure control and adjustment for continuously controlling and regulating the air pressure that enters the air passage duct to keep a fixed air pressure; an air passage nozzle having a predetermined cross section, position between the continuous pressure control and adjustment and the second open end; and a pressure sensor for detecting the air back pressure generated in the chamber after the introduction of air into the air passage duct.