Infant’s thumb is always bending and not straightening may be caused by the nervous system is not yet developed, thumb developmental deformity, congenital stenosing tenosynovitis, and so on. 1. The nervous system is not yet developed: newborn babies because the nervous system is not yet developed, limb muscles and joints are often in a state of flexion, so generally within 3 months the baby thumb will appear natural inward not straight phenomenon. 2 . Developmental deformity of the thumb: due to the influence of environmental and genetic factors, infants can appear thumb flexion can not straighten the phenomenon. 3. Congenital stenosing tenosynovitis: manifested as interphalangeal joint flexion deformity, due to the flexor tendon localized enlargement and thickening, jammed in the tendon sheath, which can produce popping, when the nodule is larger, it manifests as fixed flexion deformity, and the phenomenon of thumb bending can not be straightened. Infant thumb is always bent not straight may also have other reasons, it is recommended to go to the hospital in time for relevant examinations, clear causes and then actively treat the symptoms.