It can be roughly divided into two situations: one is that the yield curve becomes steep, the short-term yield drops sharply, and the far-end yield changes little. In this case, the duration of bond portfolio is reduced, which can generally be understood as increasing bonds with relatively short maturity and reducing bonds with longer maturity; On the other hand, the yield curve declined as a whole. In this case, the increase in the duration of the bond portfolio can generally be understood as an increase in bonds with longer maturities and a decrease in bonds with shorter maturities.