Realizing the assessment closer to real operation condition of power transformer is of great significance for arranging grid maintenance reasonably, economically and scientifically. This paper proposes the optimal combination determining weights method based on the minimum variance, which could solve the problems that the subjective weight or objective weight is assessed separately or the two weights are just simply fitted causing integral assessment one-sidedness, thus makes the weights distribute more reasonably. An eigenvalue calculating algorithm of radar chart method is applied to the transformer condition assessment, which makes the results more directly and simply and also avoids the data information omission in the process of normalization. Finally, the real data of transformer is analyzed by using the proposed method, which verifies the feasibility and reasonability of the method. This work is supported by National Natural Science Foundation of China (No. 51577046), Ministry of Education Science and Technology Foundation of China (No. 313018), Anhui Provincial Science and Technology Foundation of China (No. 1301022036), National Defense Science and Technology Plan Projects (No. C1120110004 and No. 9140A27020211DZ5102), Natural Science Foundation of Anhui Province (No. 1608085QF157), and Program for Outstanding Young Talents in Colleges and Universities of Anhui Province Project (No. gxyqZD2016207).