Abstract: A foldable stroller includes a main frame, a support unit, two fixed side units, two movable side units, and a guardrail. The main frame includes two joints, two upper connectors, a handlebar, two lower connectors, a lower bar and two front wheels. Each of the two fixed side units includes a leg, a first locking member, a first spring, a fixed block, a release member, a ramp, a pull handle, a second spring, a wheel support and a rear wheel. Each of the two movable side units includes a movable block, a press knob, an extension, a second locking member, a third spring and two braces. Thus, the first locking member and the second locking member construct a two-stage locking mechanism, to lock the stroller steadily, thereby preventing the stroller from being folded accidentally.