Currently, a large amount of defect data in relay protection devices (RPDs) is accumulated in operation. However, the defect data dependence analysis is absence and thus it could not meet the demand for further improving the management and operation RPDs. Based on 7-years defect data of RPDs in SGCC, this paper discovers the association rules (ARs) of defect data based on the Apriori algorithm. In detail, the ARs among different categories of PRDs, such as defect parts and defect causes are discovered and analyzed. Furthermore, the family characteristics of defects are illustrated, with the defect data of RPDs from different manufacturers. The analysis results show that the Apriori method can effectively reveal the hidden information in the defect data, such as the ARs between the vulnerable parts of RPDs, defect causes and other factors.