The invention relates to an atmospheric-pressure cold plasma generator system and to a method for modifying the properties of the inner surfaces of small diameter dielectric tubes. The claimed system comprises two electrodes (3 and 6), a small RF cavity electrode (3) placed inside a tube (4) to be treated and a ring-shaped mass electrode (6) mounted outside the tube (4), the two electrodes (3 and 6) being fastened and connected by means of some long and stiff conductors (2 and 7) to an adapter (5) which comprises an RF cable box. The claimed method consists in introducing a working gas at one end of the tube followed by the application of an increasing RF voltage until in the electrode area, inside the tube, there is generated a dielectric barrier discharge, after which the RF voltage is adjusted at a working value. The tube is moved by parallel displacement so that each area of its inner surface comes into contact with the plasma generated between the two electrodes placed in fixed position. By conveniently choosing the parameters such as: the working gas, the RF power, the movement rate, there can be carried out the modification of the wettability of the inner surface of the polymeric tubes, coating the surfaces with thin films, cleaning the inner surfaces with plasma, grafting chemical compounds on the plasma-activated surface.