High-frequency electron tube self-excited high-frequency oscillation instants to generate high-frequency electromagnetic field, so that the plastic internal polar molecules polarization and friction with each other to produce heat phenomenon, and then achieve the purpose of welding plastic, the welding temperature is uniform. It is applied to the sealed heat sealing of the catheters of medical PVC blood collection bags. It is mainly suitable for the blood transfusion departments of hospitals at all levels, the body collection departments and component departments of blood stations, blood banks as well as medical scientific research institutions.