A folding locking mechanism used for a step stool is disclosed. The step stool includes a stool plate, stool legs rotatably arranged on the stool plate and a locking mechanism used for locking the stool legs. The locking mechanism includes a first connecting piece and a pin, wherein the first connecting piece is fixedly connected with the stool plate and is provided with a limiting hole allowing the pin to fall therein, and the pin is limited in the radial direction and is movable in the axial direction, relative to the stool legs.