Abstract: This article combed failure modes and types of choke adapter transformer, implemented indoor circuit construction and fault data acquisition, chose the applicable fault diagnosis method, i.e. rough set theory, to obtain decision rules. On the condition that classification capability remained unchanged, plenty of redundancy features in the monitoring data table were reduced and concise decision rules were obtained. Forther more, this article used fuzzy inference to determine the applicable membership functions, implemented the fault diagnosis with high accuracy, used real data to verify the result of this fault diagnosis method. The accuracy was 92.5%, it met the actual demands.