Transmission line fault that caused by fire is closely related to weather conditions, human behavior factors and so on. In recent years, transmission line fault is increasing year by year. This paper builds a risk assessment model of transmission line fault caused by fire. This model includes two sub models: the fire occurrence probability model of the area that transmission line pass through and the probability model of transmission line fault under the fire condition. Using this model can predict the probability of transmission line fire fault according to the macro-meteorological conditions. When the fire is spreading to the line, according to the spreading model of fire, this model can estimate the time-variant risk condition of the transmission line fault. Based on the geographic conditions and vegetation near the transmission line, this risk assessment model can calculate the relationship between the fire spots and the risk of transmission line fault, so as to make a fault level distribution maps of the transmission line fault. Results in this paper can be used to formulate contingency plans for transmission line fire fault and disaster prevention measures.