A baby stroller foldable joint includes a joint mechanism having a top frame joint, a bottom frame joint and a rear wheel frame joint connecting to the top frame, the bottom frame and the rear wheel frame respectively. Each frame joint is provided with a breach at its rim, and the breaches are aligned with one another. The joint mechanism has a gear set in its interior. A control mechanism has a slider flexible to move back and forth, driving a flexible block through a linkage, for the purpose of clasping or separating the breaches of the frame joints, to accomplish expansion or folding of the frame. By aforesaid combination, expansion or folding of the frame can be simultaneous and accomplished at one go, and expansion can even be locked automatically, to make the manipulation handy and swift.