A linear actuator comprises an outer tube, an intermediate tube and an inner tube, and the tubes are all polygonal in cross section and are mounted orderly. The outer tube is disposed at the bottom thereof with a push device having a screw and a nut that are screwed with each other. The nut is engaged in the intermediate tube and is linked to move axially by the screw. The intermediate tube can be movably equipped with a pulling member, and the pulling member can link the inner tube to synchronously move in the axial direction. By such arrangements, the linear actuator allows the respective tubes to extend and retract axially relative to one another while preventing them from moving relative to one another in radial direction, thus achieving the stable and smooth axial movement.