Inclusion Criteria:  Children aged 2-5 years with ASD from Tongji hospital (both boys and girls); meets Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) and ADOS-2 diagnostic criteria; No significant abnormalities on head MRI or EEG; If psychotropic medication is required during the project, the psychiatrist must confirm that medication is stable during this period (no change in dose or type of medication); Willing to provide samples such as blood, urine and feces.  Exclusion Criteria:  Take antibiotics or antifungal drugs within 15 days prior to the project. Take probiotic products within 30 days prior to the project. Have acute diarrhea within 30 days prior to the project. Start taking new psychotropic medication within 15 days prior to the project. Have severe hearing, visual or motor impairment. Accompany with other mental behavioral disorders, such as schizophrenia, bipolar disorder. A history of Rett syndrome, chromosomal, inherited metabolic disorders and other significant somatic disorders. A history of organic gastrointestinal disorders (e.g., gastroesophageal reflux, food allergies, inflammatory bowel disease), history of intestinal surgery, intestinal obstruction, intestinal perforation, intestinal bleeding, multi-organ failure, and severe immunodeficiency disorders Have a special diet (e.g., gluten-free diet, casein-free diet, ketogenic diet, etc.) Co-morbidities in children with ASD were documented and discussed in detail by two or more behavioral developmental behaviorists, and children with ASD who required immediate rehabilitation or neurological analogs for co-morbidities were not allowed to enter this study.