A bicycle seat adjustable device includes an outer tube (10) with an inner tube (20) movably inserted therein and a piston (21) is connected to the inner tube (20). A valve control device (30) includes an outer path tube (31) in the inner tube, and a lower end of the outer path tube (31) is connected to the piston (21) and a top end of the outer path tube extends toward the inner tube. A movable piston (32) is movably mounted to the outer path tube (31). An outer oil room and an inner air room are defined between the inner tube (20) and the outer path tube (31). A valve unit is connected to the inner path tube (35) to form an inner oil room and a path which communicates with the outer oil room and the inner oil room, and controlled by the valve unit (30). The inner tube (20) and the inner path tube (35) increases the travel distance of the hydraulic oil.