The phenomenon of “deflagration” in forest fires is characterized by a sudden occurrence of high-intensity combustion with high spreading speed. Consensus hasn’t been reached about the causes of “detonation fire” so far. In this study, forest fire videos, real-time pose data, and wind speed estimation derived from KWT (Keweitai) drone, together with field research data were analyzed to characterize the spatial and temporal feature of forest fire spreading within a valley topography in Liangshan Prefecture, which killed 27 firefighters on 31 March 2019. We found that: the microclimate played a dominant role in complex terrain, and the special period of the high mountain terrain was 4:00—12:00 every day for the quiet wind period, which was the best period for the canyon forest fire fighting. The wind speed of the valley topography was active from 15:00—17:00 in the afternoon and from 20:00—22:00 in the evening, the model of relationship between inclination angle of the drone and wind speed was established y=-1.043 5+1.150 1x(y is the wind speed m·s-1, and x is the uav inclination °) . The wind speed and direction of the mountain, valley and mountainside were not uniform, and there was no positive correlation between wind speed and altitude. The peak state of the airflow velocity occurred in the middle of the valley to the depth of the valley, and there would be turbulent flow at the bottom of the valley, which provides objective and necessary conditions for the occurrence of deflagration fire. The short duration of drones was the bottleneck of forest fire monitoring.