Belt drum, tire building machine and method for forming tire tread assembly

The invention discloses a belt drum. The belt drum is used for forming a tire tread assembly, and the tire tread assembly comprises a belt layer and a tire tread layer. The belt drum can rotate to be sequentially attached to the belt layer and the tire tread layer, and the width of the tire tread layer is larger than that of the belt layer. The belt drum comprises a drum shaft assembly and a drum tile assembly, wherein the drum shaft assembly comprises a drum shaft; the drum tile assembly is arranged on the radial outer side of the drum shaft and comprises a plurality of drum tiles, and the plurality of drum tiles can move in the radial direction relative to the drum shaft; and a supporting unit is arranged on one of the plurality of drum tiles, the supporting unit comprises a plurality of groups of supporting blocks, each group of supporting blocks comprises two supporting blocks which are symmetrically arranged along the width center surface of the belt drum, and the at least one group of supporting blocks