Resilience describes the system capability of defense and recovery to extreme disasters. The transmission system, as an important part of the power system, may occur large-area power blackouts under extreme mountain fire disasters which are low-probability but highly-risk events. A resilience assessment framework considering the whole process of disaster under extreme mountain fire disasters is developed. Firstly, the influence of extreme mountain fire disaster on transmission line failure rate is quantitatively analyzed, and the mathematical relationship between the location of fire point and the line failure rate is established. Then, failure scenarios are obtained by using the system information entropy to describe the possible failure scale that caused by mountain fire disasters. Secondly, considering the geographical location of failure components, repair crews dispatch and repair time, the power transmission system restoration model, aiming at minimizing load reduction under mountain fire disasters, is established. The resilience assessment method of power transmission system considering the whole process of disasters is proposed. Finally, taking IEEE RTS-79 bus system as an example, the effectiveness of the proposed model and the resilience assessment method is verified. The numerical results show that the proposed method can effectively and comprehensively measure the influence of various factors on the elastic performance of transmission system. In addition, the effects of three typical technical measurements on improving elasticity are quantitatively analyzed, which provides a quantifiable reference for the power sector to formulate prevention and recovery strategies for extreme wildfires.