A shock absorber adapted to be mounted on a wheelchair, the wheelchair including a seat portion and a backrest portion that is connected pivotably to a rear end of the seat portion, said shock absorber being characterized by: a base tube extending along an axis, adapted to be connected to the seat portion of the wheelchair, and including a tubular wall that defines an accommodating space having an open end and that is formed with an elongated slot communicating with said accommodating space; a resilient member disposed in said accommodating space; a supporting member having an upper portion that is adapted to be connected to the backrest portion, and a lower portion that is opposite to said upper portion along the axis, that extends into said accommodating space through said open end, and that abuts against said resilient member, said supporting member being formed with a slot unit including a sliding slot that extends in the direction of the axis, and a limiting slot that extends transversely from a bottom end of said sliding slot, and that is parallel to said elongated slot of said tubular wall of said base tube; and an outer sleeve sleeved on said base tube and said supporting member, and having an inner surface and a limiting protrusion that extends inwardly from said inner surface through said elongated slot into said slot unit, said outer sleeve being rotatable relative to said base tube and said supporting member from a limited position, where said limiting slot and said elongated slot are entirely overlapped, and where said limiting protrusion is disposed at a distal end of said limiting slot away from said sliding slot, such that said supporting member is immovable relative to said base tube along the axis, to a retractable position, where said limiting protrusion is disposed at an intersection of said limiting slot and said sliding slot, such that said supporting member is movable relative to said base tube along the axis against a resilient force of said resilient member when the backrest portion pivots relative to the seat portion to incline rearwardly. a base tube extending along an axis, adapted to be connected to the seat portion of the wheelchair, and including a tubular wall that defines an accommodating space having an open end and that is formed with an elongated slot communicating with said accommodating space; a resilient member disposed in said accommodating space; a supporting member having an upper portion that is adapted to be connected to the backrest portion, and a lower portion that is opposite to said upper portion along the axis, that extends into said accommodating space through said open end, and that abuts against said resilient member, said supporting member being formed with a slot unit including a sliding slot that extends in the direction of the axis, and a limiting slot that extends transversely from a bottom end of said sliding slot, and that is parallel to said elongated slot of said tubular wall of said base tube; and a sliding slot that extends in the direction of the axis, and a limiting slot that extends transversely from a bottom end of said sliding slot, and that is parallel to said elongated slot of said tubular wall of said base tube; and an outer sleeve sleeved on said base tube and said supporting member, and having an inner surface and a limiting protrusion that extends inwardly from said inner surface through said elongated slot into said slot unit, said outer sleeve being rotatable relative to said base tube and said supporting member from a limited position, where said limiting slot and said elongated slot are entirely overlapped, and where said limiting protrusion is disposed at a distal end of said limiting slot away from said sliding slot, such that said supporting member is immovable relative to said base tube along the axis, to a retractable position, where said limiting protrusion is disposed at an intersection of said limiting slot and said sliding slot, such that said supporting member is movable relative to said base tube along the axis against a resilient force of said resilient member when the backrest portion pivots relative to the seat portion to incline rearwardly.