In the last century，the American juvenile justice system swung back and forth between welfare and punishment，care and accountability. In the 1980s and 1990s，under the growth of the juvenile delinquency rate，the American juvenile justice system，which had the characteristics of“best interests of children”and informal procedure，transformed step by step and became more rigid. Such change strongly shook the basis of juvenile justice system of which the basic idea is parens patriae，challenging the legitimacy of juvenile justice system，for it had been closing up to the adult justice system. However，the concept of juvenile justice system has grown deep in the American legal culture. Except for the juvenile justice system，thus far there is no alternative system to effectively handle juvenile delinquency.In the ruling of Miller v.Alabama and Jackson v.Hobbs in June 2012，the U.S. Supreme Court held that juvenile delinquents should not be imprisoned for life. This ruling indicated that the U.S. Supreme Court goes back to the traditional thinking of juvenile justice system to some extent. The change will have a significant impact on where the American juvenile justice system will go.