The present invention provides a baby stroller folding device comprising an upper stroller frame, having a bendable handle installed at an upper end of the upper stroller frame; a lower stroller frame, with an upper end coupled to the upper stroller frame, and having a front wheel leg of a front wheel and a bendable step pipe disposed at the bottom of the lower stroller frame; a rear wheel leg, disposed separately on both sides of the upper and lower stroller frames, and having the rear wheel installed at the bottom of the rear wheel leg, a first slide member installed at the middle of the rear wheel leg, a pull rod installed between the first slide member and the upper and lower stroller frames, and a latch rod disposed at an upper end of the rear wheel leg; a folding joint, installed separately between the upper and lower stroller frames and the rear wheel leg, for coupling and positioning the upper and lower stroller frames, and the rear wheel leg having internal and external gear discs with a notch each and selectively latched and detached with respect to the latch rod; and a link mechanism, installed between the two rear wheel legs, and having a plurality of pipe frames, slide members, link rods and a start element, and the start element being coupled to the latch rod by a steel rope, such that the baby stroller folding device can fold the baby stroller in longitudinal and transversal directions at the same time smoothly by a simple operation.