The simple portfolio investment model is given, with the HJB equation. The optimal portfolio investment problem is discussed under some given supposition, the quantitative relations are gotten between the investment strategies and riskless investment income rate and risk investment income rate are gotten. And with the quantitative relations, we study the qualitative relations between the investment strategies and riskless investment income rate and risk investment income rate. This also accounts for the effect of the falling interest rate of RMB on the national economy.