Most cases of 4 month old baby not smiling are not related to cerebral palsy, but also need to combine other symptoms and examination results to determine whether the child has autism, brain injury and other problems caused, parents need to promptly take the baby to the regular hospital for examination to clarify the cause and then deal with it. Common causes and treatment 1, less interaction, personality factors: If parents interact less with their 4-month-old baby, they may not smile, they should spend more time with their baby and interact with it. In addition, 4 months baby does not smile may also be due to personality factors, this belongs to the normal situation, no special treatment; 2, brain damage: whether the baby was born prematurely or born with a slight lack of oxygen, asphyxia, can lead to the emergence of brain damage, 4 months baby in addition to not smile, but also accompanied by large motor backwardness. Cognitive and language backwardness, need to improve cranial MRI examination, neurodevelopmental assessment to clarify the diagnosis. If there is any abnormality, timely intervention and regular review; 3. Autism: the diagnosis is generally confirmed by the developmental and behavioral characteristics of the child, which requires timely consultation at a specialist clinic and generally requires intervention training treatment and early systematic educational intervention by doctors, teachers, and families; 4. Cerebral palsy: it may not be related to cerebral palsy, which has high-risk factors causing brain injury, along with motor impairment, motor and postural development abnormalities, abnormal reflex development, abnormal muscle tone and muscle strength, etc. Timely consultation, intervention and early rehabilitation can be done, and a professional rehabilitation program can be developed by the hospital. Note parents find 4 month old baby does not smile, need to closely observe other aspects of baby’s development, such as motor, language, interaction, etc., timely medical care, at the same time parents need to have more contact with the baby, increase the baby’s sense of security and trust.