Abstract: In this paper, the author analyzed relevant factors influencing the probability (P_B) of physical damage caused by direct thunder on a building. Based on protective capability in different IP codes recommended in IEC62305 and distribution laws of lightning current probability, the author also deduced a general process used to determine the preferred value of P_B corresponding to LPL in the specification. Based on complete dependence of the deduction process on IP code recommended in the specification, thunder activity features and the feature of poor pertinence, the author proposed a method that could be used to determine the value of P_B according to the actual situation of direct lightning prevention for the building and statistical characteristics of the thunder environment at the project location. To some extent, this method improved scientific and accurate selection of P_B. At the same time, the author also made an analysis of the following factors resulting in different issues of P_B value selection: complicated physical process of lighting, scientific nature of electric-geometric model, and performance variation of protective device.