The invention discloses a steel coil uncoiling automatic locating auxiliary threading system and method. According to the threading system, a position sensor is arranged on a piston rod of a swing guide plate hydraulic cylinder, and the swing guide plate hydraulic cylinder achieves stepless speed regulation through a proportion valve; and a photoelectric proximity switch is mounted at the front end of a telescopic guide plate. The threading method comprises the steps that after an inlet steel coil vehicle completes coil feeding successfully, an uncoiling machine is used for carrying out uncoiling threading; an uncoiling machine guide plate automatic locating subsystem calculates a swing guide plate swing target position coordinate, a piston rod stretching position and a telescopic guide plate stretching point target position coordinate according to a calculation model; the swing guide plate hydraulic cylinder carries out position locating; and after the telescopic guide plate stretchesin place, an uncoiling machine motor conveys out a belt head at crawling speed. According to the steel coil uncoiling automatic locating auxiliary threading system and method, by means of the uncoiling machine telescopic guide plate, a swing guide plate, a position sensor and the proximity switch, an uncoiling machine auxiliary guide plate threading mathematic model is established in a control system, and steel coil uncoiling automatic locating auxiliary threading is achieved.