The invention provides an automatic diameter changing device for longitudinal ribs of a tapered electric pole welding machine and relates to a tapered electric pole welding machine. The automatic diameter changing device comprises a drive mechanism, a machine seat, an outer barrel fixedly arranged on the machine seat, an inner barrel arranged in the outer barrel, an inner barrel drive mechanism, screws on two end surfaces of the inner barrel and a plurality of clipping claws which are arranged on two end surfaces of the outer barrel and can guide the longitudinal ribs and conduct welding current, wherein the back surfaces of the clipping claws are provided with tooth shapes meshed with the screws; the inner barrel is connected with the clipping claws by the screws on the two end surfaces of the inner barrel; the end surfaces of the outer barrel are provided with chutes connected with the clipping claws, and the clipping claws are connected with the end surfaces of the outer barrel by the chutes; and the outer surface of the inner barrel is provided with worm wheels, the inside of the machine seat is provided with a worm rod meshed with the worm wheel, and the worm rod is connectedto the machine seat and driven by the drive mechanism. The automatic diameter changing device realizes the axial diameter change and consequently forms the tapered framework.