Financing is the prerequisite for an enterprise to start,survive and develop.To maximize its market value,an enterprise needs to take the revenue and cost derived from various kinds of financing into consideration and make out the optimum financing structure and the best ratio debt level.Based on the MM theory and equilibrium theory,the article prove the economic model of the best enterprise capital structure: P_L = P_u +[ P_u-R(d)(1-T_c)]d.Proposition 1.If P_u >R(d)(1-T_c),the debt financing can bring the good effect.Proposition 2.If P_u