Since 2018, the U. S. Government has launched a massive trade war, so it is necessary to reveal its ideological and historical root. We use the form evolution of American mercantilism as clues to do it. When comparing it with the classical form of mercantilism which was popular in Europe from the 16^th to 18^th century, we find that American mercantilism has evolved from the traditional form to the new form.　　The traditional form of American mercantilism existed mainly in the era of the rise （before 1913）. In the era, mercantilists inherited the traditional thought from British mercantilists; on the other hand, according to the socioeconomic status and geopolitical environment, American politicians put forward a set of economic development strategy, which was first proposed by Alexander Hamilton, then put forward by Henry Clay. It can be summed up as the " American System”, which consists of three mutually reinforcing parts: tariff protection, internal improvement and the national bank. Among them, the tariff protection policy is the most important one, by which the United States has achieved the industrialization path driven by domestic demand （Jia Genliang et al., 2017）, so trade protectionism has become a national memory in the United States.　　Since the collapse of the Bretton Woods System in 1971, American mercantilism has developed its new form. Compared with the classical form, the new form is really a distorted expression. On the one hand, trade deficit has become a main fulcrum to maintain dollar hegemony, while the classical form emphasizes on trade surplus. On the other hand, dollar hegemony becomes the main means of depriving other countries’ wealth. Although one is in the era of metal currency, the other in the era of credit currency, dollar hegemonism is largely as same as bullionism in essence, which is the earlier form of classical mercantilism. Nowadays, " double deficits” or the coexistence of trade deficit and fiscal deficit is the most important feature in American economy, and the trade protection policy is the most important means to maintain American hegemony.　　Historically, there is a deep economic strategy root for the evolution of American mercantilism. In the era of the rise, the United States hoped its national strength can be improved through the development of manufactures by the tariff protection policy and domestic market competition. Since 1971, the United States hoped its dollar hegemony can be recast to intensify American hegemony, so a series of abnormal economic policies and phenomena appeared. Since then, despite the form of American mercantilism has changed completely, the old ideas in classical mercantilism such as " zero-sum game” are still in vogue because of the strategic demand of American hegemony. The above analysis implies that the United States would take such strategic actions as a trade war to suppress its most important strategic rivals, while it inevitably incurs the result of " lose-lose”.