This paper first indicates that saving equals to the liquidity preference plus the supply of loanable funds and the liquidity preference is just opposite to the supply of loanable funds. Meanwhile, the paper proposes a new model in which interest rate is determined by the investment demand curve and the symmetrical curve of the liquidity preference curve about Y axis. On such basis, the paper notes that the existence of liquidity preference makes effective demand always deficient. Thus market failure becomes the norm and the government is obliged to take aim at the interest rate which is determined by the desired investment and desired saving. So far the paper has thoroughly clarified how interest rate is determined and constructed a new an...