Ultra high voltage(UHV) transmission lines across forest areas and straw crops, and the occurrence of wiforest fire threats the reliability and stability of the power system. Considering the influence of the fire temperature and the carbonized small particles, ashes and smoke on the air gap insulation of transmission line, and the height of wildfires is taken as the correction factor of air gap breakdown voltage, the calculation model of breakdown probability of UHV transmission line under the condition of mountain fire is established then. The simulation results show that the breakdown probability of UHV exhibits a "S" growth pattern with temperature and smoke density. And the effect of fire height on insulation of the air gap is similar to that of temperature. Under the same fire condition, the phase-to-ground breakdown probability of UHV is much lower than that of Extra high voltage(EHV). At the same smoke density, the phase-to-phase breakdown is more likely to occur at UHV. But because of the lower smoke density under the same fire condition, the actual situation may be opposite.