查查维基百科全书就知道了嘛。。。

http://en.wikipedia.org/wiki/American_nationality_law#Dual_citizenship

Based on the U.S. Department of State regulation on dual citizenship (7 FAM 1162), the Supreme Court of the United States has stated that dual citizenship is a "status long recognized in the law" and that "a person may have and exercise rights of nationality in two countries and be subject to the responsibilities of both.