After summarizing the reasons of subprime crisis happening in related literatures,the authors construct the model to analyze the reason of currency liquidity risk’s formation based on the bank reality’s principalagent question. They get the conclusion: The proxy question causes the bank manager’s payoff incentive to reduce the interest rate and makes loans excessively. In the situation that the trustee introduces the audit supervision,when there is heightened macroeconomics risk,investors reduce direct investment and hold more bank deposits. This‘right to quality’leaves banks flush with liquidity,attenuates manager’s penalty restraint,then causes the manager to make loans excessively and results in asset price bubbles,at last increases systemic risk.