The calculation of transmission line’s real-time fault probability is an important part of real-time risk assessment and reliability calculation of power system. In order to calculate the real-time fault probability value of transmission line, first of all, according to the online monitoring data of the transmission line’s insulator, conductor, lightning arrester, tower, and ground line, the quantitative model of each monitoring parameter is established respectively. Then the theory of unascertained rational numbers is used to determine the weight coefficient of each layer, and combining with fuzzy assessment method, finally a real-time and hierarchical fault probability assessment model of transmission line is established. Through analyzing the failure statistics data of southern power grid, it is found that bad weather has become an important factor causing the fault of transmission line. Considering the effects of bad weather on transmission line’s fault probability, this paper puts forward and sets up a real-time evaluation model of transmission line’s fault probability considering both transmission line’s on-line monitoring data and the current meteorological conditions. The example shows that the proposed transmission line’s real-time failure probability assessment model is reasonable and effective.