Under the condition of the unlimited resource of pledge and floatable interest rate, the optimal contract between a bank and enterprise is studied from both sides of information symmetry and information asymmetry after credit. Whether information symmetry or not after credit, a separating equilibrium between high risk enterprise and low risk enterprise can be realized with different combination of pledge and interest rate in competitive market. In monopolistic market a contract combination can also be used to force high risk enterprise to leave credit market. Compared with common screening mechanism, this kind of screening mechanism can help to realize larger total return whether information symmetry or not after credit.