Mountain fire disaster is one of the important factors that threaten the safe and stable operation of power grid. In order to effectively prevent mountain fires, this paper proposes an evaluation model against mountain fire based on AHP. On the basis of the three aspects: the historical fault of the transmission line, the real-time monitoring data, the weather forecast information, the hierarchical structure is constructed. Combined with the operation and maintenance data of Guizhou power grid and the expert experience, the judgment matrix is built up to make a feasibility judgment and to get the relative weights and the overall weights. With the deduction table given by the expert system, the actual score was calculated. Then the paper assessed the risk of mountain fire of the transmission line and made the suggestion to prevent the mountain fire. Actually the evaluation model improves the efficiency of mountain fire control and the stability of the power grid.