The normal angle of baby’s hip joint refers to α＞60 degrees and β＜50 degrees under ultrasound. Baby hip joint angle is a normal indicator for assessing hip dysplasia, generally under ultrasound, normal is divided into four types, type i is normal, just follow up regularly. Type Ⅱ a-b hip immaturity (age greater than 3 months 60 degrees <α<50 degrees β<77 degrees ), no treatment, regular follow-up. Type II c-d acetabular dysplasia ( 49o<alpha<43o degrees beta>77 degrees), requires treatment with a child sling or soft brace. Type III hip subluxation or low dislocation Ultrasound findings ( α<43 degrees β>77 degrees) require treatment. So knowing the hip angle can determine the degree of hip dysplasia.