Private equity funds usually invest in enterprises, including seed funds and growth capital, that is, venture capital funds. It also includes direct investment funds and buyout funds for enterprises in the expansion period, LBO funds participating in MBO/MBI, and bridge funds for enterprises in the transition period (mezzanine/bridge) or pre-IPO (Pro-IPO) are also the investment scope of private equity funds, that is, all equity investments obtained before the company goes public belong to private equity. This industry came into being in the early 1980s in the United States and continues to develop. In the 1990s, it became a financial industry that played an important role in American economy and enterprises. Private equity funds can also be classified according to the fund size. Usually, the bigger the fund, the later the development of the investment enterprise, that is, the acquisition fund or the bridge fund. Small funds are venture capital or venture capital funds, and the most complicated is the mid-end market capital fund-mid-end capital, which has both high-end investment and low-end investment. Different funds in the middle market have different investment objects and styles, unlike venture capital or acquisition funds, which have clear goals. The following table shows the average size of private equity investment funds of different sizes in the United States and the average investment amount invested in all projects.