The invention discloses an automatic edge cutting device for a friction welding machine. The automatic edge cutting device comprises an edge cutting device body installed in the friction welding machine; the edge cutting device body comprises a transmission component and an edge cutting component; the transmission component comprises a driving gear, a driven gear, a transmission shaft, bearing seats, supports and a first oil cylinder; the driving gear is mounted on a main shaft of the friction welding machine; the driven gear is mounted at one end of the transmission shaft and is opposite to the driving gear in position; The two bearing seats are mounted on the transmission shaft; an external thread segment is arranged at the position, located between the two bearing seats, on the transmission shaft; the two bearing seats are slidably connected with the two supports, and the two bearing seats are both connected to the first oil cylinder; the edge cutting component comprises a moving seat, a knife holder connected to the upper portion of the moving seat and a cutting knife mounted on the knife holder; and the moving seat is arranged at the position of the external thread segment ofthe transmission shaft in a sleeving manner through an internal threaded hole of the moving seat. The automatic edge cutting device has the advantages of being simple in structure and low in manufacturing cost and being capable of achieving automatic cutting of flash.