Through analysis of small and medium-sized enterprise private bond defaults，this paper finds that guarantee of bond is inhibited and risk allocation is invalid，therefor these guarantee problems increased bond default risk. Investors and bonding company virtually constitute of principal- agent relationship in bond market. The apply of Parameterized Distribution Formulation can solve the problems：choosing bonding company of risk neutral，improving the bonding company’s effort，and bonding company’s subjective efforts is affected by its objective ability and effort cost. The conclusion of the paper can also be used to OTC market.