A adjustable back support apparatus is provided, including: a back support, including a first joint portion, the first joint portion having an elongate slot and a fixation hole communicated with the elongate slot, the fixation hole including a first hole； a shoulder support, including a connection portion and a second joint portion connected with the connection portion, the second joint portion having through holes, the second joint portion movably inserted into the elongate slot so that one of the through holes of the second joint portion being movable to correspond to the fixation hole； an engaging member, swingably disposed on the first joint portion, the engaging member having a protrusive buckle, the protrusive buckle disengageably buckled within the first hole and one of the through holes, at least a part of the engaging member being movable between a blocking position and a release position.
