The belt ply feeder and a feeding method include: when a belt ply is conveyed to the front end of a conveying forming board, a force is exerted by a perpendicular adjustment device to drive a conveying forming board bracket to swing upward and rotate around a pivot shaft of a rack to positions at which the upper surface of the conveying forming board is tangential to the periphery of a belt ply drum. According to this arrangement, the belt ply is tangential to the belt ply drum all the time and a belt ply is in firm contact with a belt ply drum regardless of the size of the belt ply drum when wrapping the belt ply around the belt ply drum. Accordingly, the quality of tire shaping can be improved.