Through the four stages of development，the US OTC market has formed a modern，multi-level market system. For different levels of the OTC market，there are different access standards and regulatory requirements. In terms of trading system，the market-maker is the core，but the NASDAQ market has evolved from a purely market-maker system to a hybrid trading system. As a whole，the US multi-level OTC market runs successfully，in the process of meeting the need of corporate finance and securities trade，the OTC market promotes enterprise development and thus indirectly supports the US economic growth.