Monetary fund is an open-end fund that collects idle social funds, is operated by fund managers and kept by fund custodians. Special investment in risk-free money market instruments, different from other types of open-end funds, has the characteristics of stable income, high security, high liquidity and "quasi-savings". The investment scope of money market funds is a variety with high safety factor and stable income, so for many enterprises and individuals who want to avoid the risks in the securities market, money market funds are a natural haven, which can not only obtain the income higher than the interest of bank deposits, but also ensure the safety of the principal. Generally, there is an upper limit for the subscription of money funds, and the upper limit of each fund is different.