The invention discloses an installing and sizing die for an automobile headrest and relates to a die. The installing and sizing die comprises an installing support and an installing and sizing die body connected to the installing support. The installing and sizing die body comprises a lower die, an upper die, a left side and a right die. The upper die is rotatably connected to the position, close to the rear side, on the lower die. The left die can be connected to the position, close to the front side, of the left end of the lower die in a sliding manner, and the right die can be connected to the position, close to the front side, of the right end of the lower die in a sliding manner. When the upper die, the left die and the right die completely cover the lower die, a closed sizing cavity is formed. A left guide rod is connected to the outer wall of the left side of the lower die, and a right guide rod is connected to the outer wall of the right side of the lower die. The outer side of the left end of the left die extends leftwards to be provided with a left connecting plate. The lower surface of the left connecting plate is connected with a left sliding block arranged on the left guide rod in a sleeving manner. The outer side of the right end of the right die extends rightwards to be provided with a right connecting plate. The lower surface of the right connecting plate is connected with a right sliding block arranged on the right guide rod in a sleeving manner. A locking device capable of locking the left die and the right die together is further arranged on the front side of the left die and the front side of the right die. By the adoption of the installing and sizing die, foam materials can be directly sized in an inner cavity of a decoration outer cover, the production cost is low, and the product quality can be effectively ensured.