Repetitive and stereotyped behaviors are the core symptoms of ASD and the indicators of the severity of ASD, which seriously affect the daily life, learning, and social interaction of children with ASD. The reasons for the formation of repetitive behaviors in children with ASD have not been clear. Through long-term clinical observation of children with ASD, the study found that part of the stereotyped behavior, which is the interests and activities of children with ASD, can be changed. By finding the regularity of the stereotyped behavior, the stereotyped behavior will be reduced or stopped. The study proposed a cognitive mechanism hypothesis for the reduction of stereotyped behaviors: some stereotyped behaviors have potential functions, such as finding regularity. ASD finds the regularity and improves the accuracy of the knowledge of the regularity by repeating the operation. After finding regularity, ASD accomplishes its purpose, forming functional behaviors and reducing stereotyped behaviors. With the accumulation of time, the known regularities are gradually integrated, which shows the tendency of more complete cognition, more functional behavior, and less rigid behavior. This hypothesis provides a new perspective to understand the stereotyping of ASD. The study views stereotypy as non-disease, potentially functional and mutable, which can expand the understanding of ASD and help parents better understand and develop its advantages, with important potential social implications.