An adjustable back cushion structure comprises an adjusting device, which has three portions, a positioning portion, an adjusting portion and a joining portion, the positioning portion is disposed a block portion thereon, a plurality of fixing portions are disposed between the positioning portion and the adjusting portion, a joining surface is on the top end of the joining portion; and a cushion, which is a member covered by a loop surface, filler is filled into the cushion, any of the surface of the cushion is able to to join the joining surface of the adjusting device.