Pre-pruning means setting an index in the process of tree growth. When the goal is achieved, it will stop growing. This will easily lead to a "horizon reduction". This means that when a branch is stopped and node n becomes a leaf node, it will cut off any possibility of a "good" branch operation for its subsequent nodes. It is not strictly stated that these stopped branches will mislead the learning algorithm, leading to a maximum reduce the pollution of trees near the root node.