Abstract: Described in the paper are the configuration of and application requirements for the high-speed steel tube flaw detecting line. Disadvantages of the conventional pneumatic pressing type mechanism are analyzed. A magnetically-fixing mechanism is designed based on the well-selected magnet structure and relevant parameters. Being particularly applicable to the tube detecting line with automatic charging/discharging functions, the said magnetically-fixing aggregate features such advantages as controllable magnetic force, quick response, high efficiency and long service span, etc. The experiment result shows that the said magnetically-fixing mechanism is satisfactory to the requirement for applied magnetic force for high-speed detecting line, being feasible and applicable.