Early education for children at risk or with mental retardation is called “early intervention”. Early intervention is an organized and purposeful educational training activity that enriches the environment (provides stimulation). It is appropriate for children before the age of 5 to 6 years (mainly before the age of 3 years) whose development is slightly off (or behind) normal or at risk of being off (or behind) normal. Through this kind of education, it is expected that the intelligence of these children can be improved, and they can acquire certain life abilities and skills, so that they can grow up to school age and be better able to receive special education or normal children’s education.There is also a critical period of intellectual development for children with PKU.In 1992, the China-Japan Friendship Hospital in Beijing, in cooperation with the Department of Education of the Beijing Normal University, was the first to try to treat children with PKU who were already suffering from intellectual disabilities due to late detection. In 1992, the China-Japan Friendship Hospital and the Department of Education of Beijing Normal University cooperated in the first attempt in China to provide early intervention for children with PKU who had intellectual disabilities due to late discovery, along with dietary treatment. The principles of intervention: taking the existing intellectual age level of the child as the starting point for training; focusing on family training, mobilizing parents and other members of the family to participate; adopting a small-step training program, breaking down the training program into small steps, and arranging them according to the actual level of the child, in order to formulate an individual program; emphasizing life as the basis, using the family as the base, and integrating the life training content with games, and reinforcing them anytime in life. After 6 months of training, the developmental quotient and annual intellectual age growth rate of the intervention group were again significantly higher than that of the non-intervention group, and fine motor and prehension also improved. It shows that early intervention has obvious effect and provides some experimental basis for comprehensive early intervention for PKU children with mild, moderate and severe mental retardation.