A locking mechanism for opening and folding a baby stroller and a locking method therefor are revealed. The baby stroller includes an outer tube of a handle, a front wheel frame, and a rear wheel frame, each of which is arranged with a pivot member for connection. An inner tube is mounted and moveable in the outer tube and a push button for folding is arranged therebetween. A locking hook and a locking rod are pivotally connected to the pivot member of the outer tube and the pivot member of the rear wheel frame respectively. The handle, the front and the rear wheel frames are extended and fixed by the locking hook hooked onto the locking rod. While operating the push button for retracting the inner tube into the outer tube, the locking hook is released from the locking rod so that the baby stroller is folded.