At present，the United States is the only developed country which maintains fast population growth，and has the third largest population，only after China and India. As the world’s largest economy that tops all countries in terms of comprehensive strength，the developing trend of the US population and the US government’s policy on population have great influence on both the U.S. and the whole world. This paper elaborates on history，current situation and characteristics of the US population development as well as the US government’s policy on population，analyzes the causes for the US population growth，discusses major issues regarding current US population，and expounds the impact of the US population growth on the U.S. and the international community from political，economic，social and ecological perspectives.