Based on the recovery voltage test datum, the rough set theory is used to evaluate the insulation status of the transformer and a transformer insulation status evaluation system is established. Firstly, the characteristics of the recovered voltage method and the extended Debye equivalent relaxation circuit are selected as the evaluation features. Then, the fuzzy C-means clustering algorithm is used to divide the quantities of evaluation features into fuzzy indicators and generate the insulation status decision table. The discrepancy matrix performs attribute reduction on the insulation status decision table. Finally, the blind reduction value reduction algorithm is used to reduce the attribute value of the reduced decision table, and a complete reduction decision table is obtained. On this basis, the transformer insulation status evaluation rule table is established. The example analysis proves that the proposed evaluation method can effectively evaluate the internal insulation status of the transformer. This work is supported by National Natural Science Foundation of China (No. 61174117).