Accurate assessment of the power transformer condition is a basis for reasonable maintenance strategies, and is of great significance for smart grid development. Aiming at the transformer complex fault mechanism and the difficulty of objective assessment, a method of transformer condition assessment based on A.J.Klee method and extension hierarchy is proposed, which transforms transformer multi-parameter synthetic evaluation into single-object decision. First, the hierarchical index system is established, and the condition grade is divided. The weight of the index is calculated by A.J.Klee method. The comprehensive evaluation level is determined by the extension hierarchy, and the results are compared with the Fuzzy Analytic Hierarchy Process (FAHP). The feasibility of the method applied to the evaluation of transformer status is verified by an example, which provides a new assessment strategy for transformer insulation condition maintenance. This work is supported by National Natural Science Foundation of China (No. 51577046), State Key Program of National Natural Science Foundation of China (No. 51637004), and National Key Research and Development Plan "Important Scientific Instruments and Equipment Development" (No. 2016YFF0102200).