In 2018，five American health insurance groups entered Fortune Global 500 in the background of the inflation of medical costs in the United States and the reform of the public-private partnership system of basic medical insurance in the United States. The report holds that American health insurance groups assume three major functions in the medical service system and achieve functional expansion，which is the key and reflection of becoming bigger and stronger. In the premium income composition of large health insurance groups，the premium income of government medical insurance business occupies a large share，providing supports for them to assume important functions in the medical service system. In addition，the scale economy effect is also the reason why health insurance groups entered Fortune Global 500.