The conventional methods for wildfire risk assessment of transmission lines only consider few hazard indicators and largely depend on subjective experiences. This paper proposes a wildfire risk assessment model for transmission lines based on matter-element extension. First, a total of 15 risk sub-factors in four classes of anthropic, topographical, meteorological and transmission lines factors are assembled to construct a wildfire disaster risk assessment index system for transmission lines. Then, the analytic hierarchy process combined with the entropy theory is to evaluate combination weights of indexes, in which subjective and objective weights are combined. Next, a matter-element extension evaluation model is established to measure the correlation of risk indicators. Finally, case studies of transmission lines in a southern province of China are analyzed. And a comprehensive risk assessment for wildfire disasters is carried out. It provides a basis for managers to carry out differentiated wildfire prevention and control work.