The dissolved gas analysis technology of the transformer has become one of the most common and important technologies of the transformer fault diagnosis. But there are a lot of uncertainty factors in the fault diagnosis. To resolve this question, this paper proposes a method of basic trust distribution function research which based on the Duval triangle method. This method based on the theory of Duval triangle and uses the interpolation method of finite element three-node triangle to construct basic trust distribution function. Finally the feasibility and availability of this method is proved by practical calculation.