The invention relates to an automatic polishing device for the inner surface of a drill stem welding seam, and is characterized in that the device comprises a base (25), a grinding machine (20) and a bracket (5); the base (25) is longitudinally provided with slide rails (24); the grinding machine (20) is arranged on the slide rails (24); the rear end of the grinding machine (20) is connected witha reciprocating transmission mechanism (12) which is connected with a servo motor (11); the servo motor (11) is installed on the base (25); the grinding machine (20) comprises a supporting seat (26),a sliding table (17), a traverse air cylinder (16), a rotary table (6), a swinging air cylinder (8), a lead screw (1), a lathe bed (27), a grinding rod (2) and a motor (7); the bracket (5) is installed in front of the base (25), and comprises a supporting frame (29), a bracket table (28), rollers (4) and a lifting air cylinder (15). The device can automatically polish the inner surface of the drill stem welding seam so that the noise in the working process of the device is low and the dust generated in the polishing process can be discharged and collected by the other end of the pipe body, thereby improving labor environment, reducing labor intensity, and having high working efficiency.