The invention relates to a tire building machine belt layer drum and a radial expansion and contraction method thereof. A servo motor is adopted as an axial driving device; with motion mode conversion, drum tile radial expansion and contraction changes are caused with a mechanical transmission mode, such that the radial expansion and contraction distance of each drum tile can be precisely controlled. Therefore, belt layer drum diameter range adjustment precision can be improved. The belt layer drum radial expansion and contraction method comprises the steps that: at least a set of drum tiles which can be radially expanded and contracted, and a conical sleeve which can be subjected to synchronous axial rotation with the drum tiles are radially sleeved on a main shaft of the belt layer drum. A thrust device driven by the servo motor is axially arranged along the main shaft. The thrust device comprises a first push rod laterally connected with the conical sleeve. The first push rod rotates synchronically under the driving of the conical sleeve, such that the first push rod can push and pull the conical sleeve along an axial direction, and drum tile radial expansion and contraction can be realized.