The United States is always regarded as the base of the economic liberalism for a long time, but her trade policy has turned to protectionism after the subprime mortgage crisis. This article reviews the history of American economic development and the economic thoughts of the American school and points out that protectionism was the usual choice of American trade policy at the early time after her independence and that free trade is the choice after her economic hegemony was built. Thus, American protectionism after the crisis has its historic causes, should not be viewed as the basic change of her trade policy from free trade to protected trade, however, American economic policy returned to the tradition of its protectionism.