Mountain fires pose a threat to the safe operation of transmission lines. Firstly, the historical pattern of mountain fire incidents in Hunan province from 2018 to 2020 is reviewed. On this basis, a database of mountain fire incidents is constructed. One?hot encoding technique is introduced to numerically process textual features such as the vegetation type along the transmission lines. Then, XGBoost technology is utilized to build a mountain fire risk assessment model for transmission lines. Aiming at imbalanced samples between forest fire incidents and normal operations, based on a cost?sensitive mechanism, a weighted objective function is presented to mitigate the problem of overlooking mountain fires caused by sample imbalances. Finally, the proposed mountain fire risk assessment model for transmission lines is tested using the forest fire incidents in Yongzhou City, Hunan Province, from 2020 to 2021 to validate its effectiveness.