The jamming effect of airborne towing decoy is very available to air-defence radar and seriously affects the operational ability of air - defence missile. So, it is a necessary work to design and develop the towing decoy system for air defence training and apply it into the tracking training of air-defence radar. The jamming mechanism and jamming effect of the towing decoy to mono-pulse radar are analysed. According to the jamming equation of the towing decoy, the utilization requirement and main design factor of the towing decoy system for air-defence training are analysed and discussed. The construction and realizing method of the system are presented.