The invention discloses an automatic opening flattening machine for a thin-walled stainless steel pipe. The automatic opening flattening machine comprises a machine cabinet, and an electrical controldevice and an industrial personal computer are mounted in the machine cabinet; a base is mounted at the top of the machine cabinet; two mutually parallel linear slide rails are arranged on the base; ascrew rod supporting seat is mounted on the base; a screw rod is arranged on the screw rod supporting seat in a sleeving manner; one end of the screw rod is connected with a screw motor; a transmission box is arranged on the screw rod supporting seat; a driving motor connected with the transmission box is mounted on the transmission box; a rotating spindle is mounted in the transmission box; oneend of the rotating spindle is connected with a decelerator in the transmission box; the other end of the rotating spindle stretches out of the transmission box; a rotating lathe tool is mounted at the stretching end of the rotating spindle; and a pipe fixture is mounted on one side of the rotating lathe tool. According to the automatic opening flattening machine, the problems that pipe openings deform, the security risk is high, and efficiency is low when traditionally, devices like a cutting machine/angle grinder are manually utilized to flatten the openings are solved.