Parameter 〖WTBX〗P〖WTBZ〗c refers to the reference value of electrical and electronic system failure probability in buildings resulted from lightning strokes in the evaluation of lightning disaster risk, and the selection methods of the factor directly influence the accuracy of assessment results. Based on the concept of 〖WTBX〗P〖WTBZ〗c, an analysis is made of 〖WTBX〗P〖WTBZ〗c, and the quantitative calculating methods are discussed. It is concluded that while choosing the parameter, the rationality of SPD design and the limitations of SPD protection should be considered according the relevant national stands, and also the statistical analysis should be conducted of the historical data of local lightning locations to determine the probability distribution of the lightning current peak. The selection of 〖WTBX〗P〖WTBZ〗c should be carried out to ensure the design of SPD is scientific and effective. The protection probability is determined through the analysis of the probability of lightning current peaks, to determine quantitatively the values of 〖WTBX〗P〖WTBZ〗c.