Abstract: To prevent polarity errors in relay protection systems that could affect the normal operation of the equipment, this paper proposes a method for active, same-frequency, and phase-detection of relay protection polarity based on vulnerability indices and association rules. This method calculates the vulnerability index for active in-phase polarity detection in relay protection. Using the Apriori association rule mining algorithm, it constructs an indicator system for active in-phase polarity detection in relay protection, determines the comprehensive and individual indicators for in-phase polarity detection, and calculates the degree of association between them as well as the constant weight coefficients for the individual indicators; Based on this, after obtaining the variable weight coefficients for the comprehensive indicators, the calculation results are combined with the vulnerability index to derive the comprehensive score for the detection indicators. Test results show that this method can calculate the vulnerability index during the polarity-active, same-frequency phase-determination process of relay protection devices; determine the strong correlation between comprehensive and individual indicators; and obtain the comprehensive score results for each comprehensive indicator. The detection error results for the polarity-active, same-frequency phase-determination of relay protection devices are all within 0.5°.