The yield curve is a graph showing the yield of a group of bonds or other financial instruments with the same currency and credit risk but different maturity dates. The vertical axis represents the rate of return, and the horizontal axis is the time from maturity. Rate of return refers to the investment rate of a single project, and interest rate is the total level of all investment returns. In most cases, the rate of return is equal to the interest rate, but there is often a deviation between the rate of return and the interest rate, which leads to the inflow or outflow of funds in a certain field or at a certain time, thus bringing the rate of return closer to the interest rate. During the period, the trend of bond yields may be inconsistent, and three yield curves may be formed: upward, horizontal and downward.