Internal expanding tool achieving automatic centering

Provided is an internal expanding tool achieving automatic centering. A rotary guide ring is arranged on a telescopic centering body in a sleeving mode, and the rotary guide ring and the telescopic centering body are in clearance fit. The spherical surface ends of telescopic screws in all telescopic bodies penetrate through guide holes located in the circumferential surface of the rotary guide ring respectively to be mounted in a sliding groove in the circumferential surface of the telescopic centering body. A worm is fixed to the center of the telescopic centering body through a bearing. A worm wheel/gear assembly is mounted on the disc surface of the telescopic centering body through a bearing. A turbine body in the worm wheel/gear assembly is engaged with a worm, a gear body of the worm wheel/gear assembly is engaged with an inner gear ring section of the end face of the rotary guide ring, and relative rotation is generated between the worm wheel/gear assembly and the inner gear ring section. During work, all supporting blocks can stretch out and draw back at the same time, the locating center does not change all the time, and automatic centering can be achieved. The center of an expanded barrel or shell is determined, and rounding is achieved while expanding of the barrel or shell is achieved.