A method and device allows dynamic mask welding with online position adjustment in the high-speed welding of structured and endless plastic films. By a transparent driven mask roller provided with masks being arranged in a passive pressure roller surrounding the mask roller, detection can take place optically, and the mask roller and consequently the mask are synchronized with the film speed. Adjustment is detached from a film transport mechanism and does not influence the welding process and in this case, above all, the speed. High film transport speeds in the range of up to 1000 mm per second can be employed.