Babies like to put their legs up when they sleep for two reasons, related to physiological and pathological factors. Physiological factors include habit. Pathologic factors are related to organic damage. Infants from 4 months to 8 months of age have relatively developed muscles and are in the second flexion period at this age, which is a physiological factor for babies to tilt their legs upward. If the infant has ischemic-hypoxic encephalopathy at birth, then most of them will be manifested as muscle tension is too high, it is easy to appear such a manifestation of the form, and at the same time, if the infant 10 months after the leg joints dislocated, soft tissue injuries, it is also easy to appear legs up, need to consult a doctor in time to diagnose and treat. In conclusion, it is recommended that babies with developmental anomalies seek medical attention in a timely manner.