The invention relates to a built-in oil cylinder mechanism of a mold for a plastic pipe fitting, which comprises an upper top plate and a lower bottom plate of the mold, wherein the upper top plate is connected with a fixed mold plate, a mold leg is mounted on the lower bottom plate, an ejector plate is arranged in the mold leg, an ejector rod is connected on the ejector plate, a movable mold plate is mounted on the mold leg, the molded plastic pipe fitting is arranged between the movable mold plate and the fixed mold plate, and a loose core mandrel is arranged in the plastic pipe fitting. The built-in oil cylinder mechanism is characterized in that a cylinder body of a loose core oil cylinder is mounted in the loose core mandrel, a fixed support is mounted on the side wall of the movablemold plate, and a piston rod of the loose core oil cylinder is connected with the fixed support. In the scheme, the cylinder body of the loose core oil cylinder is mounted in the loose core mandrel, and the piston rod of the loose core oil cylinder is connected with the fixed support on the side wall of the movable mold plate. When a core is pulled, the cylinder body can drive the loose core mandrel to slide, the actions are stable and reliable, the space occupied by the loose core oil cylinder is simultaneously reduced, the volume of the mold is reduced, the energy is saved, and the cost is lowered.