The geographical environment around power transmission channels which pass through the forest is complicated and forest fire occurs frequently. In order to study the problem of risk assessment of power transmission channels in forest region, meteorological data of multiple areas in Liangshan Autonomous Prefecture, Sichuan Province where two power transmission channels of an important load pass through is collected and analyzed according to the chronological order. The occurrence probability of forest fire is calculated by forest fire danger weather ratings, and the fault probability of transmission lines under forest fire condition is calculated by breakdown characteristics of long air gaps theory. The temporal and spatial risk evaluation model of power transmission channels under forest fire condition is built. The validity of the proposed method is verified through a case analysis. Besides, the operation conditions of transmission lines are analyzed after power flow transferring due to one power transmission channel breaks down. Finally, some risk-mitigation measures are proposed according to the results. This work is supported by Key Project of Smart Grid Technology and Equipment of National Key Research and Development Plan of China (No. 2016YFB0900600) and Major and Basic Project of State Grid Foresight Science and Technology (No. SG20141187).