The invention provides a forming die for a ball liner. The forming die for the ball liner can assist a worker to spread a rubber sheet on a die middle plate, and comprises an upper cutter point, a lower cutter point, a left middle plate and a right middle plate. The left middle plate and the right middle plate are of the same structure; the left middle plate is of a hollow structure and is internally provided with a sliding block capable of sliding in the width direction; pull ropes are fixed to the front end face and the rear end face of the sliding block; and two thin seams corresponding to the two pull ropes are formed in the front end face and the rear end face of the left middle plate to allow the pull ropes to pull the sliding block to slide. At least three rows of air suction holes are evenly formed in the upper end face of the left middle plate in the width direction; seamed edge clamping mechanisms are fixedly arranged at the positions, close to the top ends, of the front end face and the rear end face of the left middle plate; and the seamed edges, towards the left middle plate, of the seamed edge clamping mechanism can push out clamping sheets parallel to the seamed edges in an extensible mode. According to the forming die for the ball liner, wrinkles before production of the ball liner can be effectively flattened, wrinkles generated during the production are also prevented, the product yield rate is improved, and the production cost is lowered.