The invention discloses a backfill type stir friction spot welding device, and relates to the technical field of stir friction welding. A jacket is fixed at the lower end part of an outer shell; an arm cover is arranged in the jacket; a stirring needle is arranged in the arm cover; a hollow shaft motor is provided; a hollow arm cover shaft is arranged in the hollow shaft motor; a stirring needle shaft is arranged in the arm cover shaft; the arm cover shaft is connected with the arm cover; the stirring needle shaft is connected with the stirring needle; a first ball screw with a screw rod as a hollow screw rod is arranged above the arm cover shaft; the stirring needle shaft is inserted in the screw rod of the first ball screw; a first bearing frame is arranged at the lower end of the screw rod of the first ball screw; the upper end part of the arm cover shaft is mounted on the first bearing frame; a second ball screw is arranged above the stirring needle shaft; a second bearing frame is arranged at the lower end of a screw rod of the second ball screw; the upper end part of the stirring needle shaft is mounted on the second bearing frame; and a first power source and a second power source are respectively connected with nuts of two lead screws. The backfill type stir friction spot welding device has such advantages as simple structure, high system stability, high welding precision, good effect and long service life.