An adjustable positioning structure of a shaft member contains an inner tube including a plane having plural holes, an outer tube including a cutout to receive the inner tube, an adjustable positioning device including a fixing seat, and a retaining block formed in a twisted piece shape and axially defined between two extending sides of the fixing seat. The retaining block includes a pressing portion and a bore fixed on one end thereof, and includes a lock tab extending outward from another end thereof. A stop switch is covered by the fixing seat and includes a shoulder to be inserted in the bore to stop the retaining block to be pressed and to operate. An resilient element is defined between the retaining block, and the stop switch to contact with the retaining block, so that the lock tab is retained in one of the holes.