Secondary circuit failure of voltage and current will lead to measurement distortion of relay protection which affects its action performance. Microprocessor-based protection has strong self-checking capability, but mismatch between failure detection and action discrimination characteristics of relay protection will lead to unwanted trip. In order to improve its fault-tolerant ability, an evaluation method of match degree between failure detection and action discrimination characteristics is proposed in the paper. Detection blocking zone, abnormal measurement zone and protection action zone are introduced into the action characteristics plane and the effect of their relationship on the performance of relay protection is analyzed based on Venn diagram. The result indicates that non-detection zone can be eliminated to prevent unwanted trip only when detecting blocking zone contains the intersection of abnormal measuring zone and protection action zone. The effectiveness of this method is verified by an actual case.