Abstract: The length-adjustable gas cylinder is disclosed, which includes a cylinder that reciprocates in an inner through hole of a slide member of an outer container; a push bar and a push support that are installed at an upper side of the interior of the cylinder for performing a gas sealing and an opening and closing operation of the gas flow path; a gas flow path pipe that is fixedly inserted into the lower side of the push bar and performs a role of the path communicated so that the gases of the chambers X and Y communicate and has a gas flow hole capable of determining the maximum move-up distance of the cylinder; an integral piston assembly integrally formed with the piston part and the piston rod part; and a clip formed of a synthetic resin that fixes the piston rod part.