The invention discloses a transmission line mountain fire early-warning method based on an image and gray prediction. The method comprises the steps that state standards of mountain fire behaviors are established, the distance between the mountain fire behavior at the highest position and a transmission line serves as a clearance distance, and at least four warning standards are obtained through division according to the clearance distance; a grayscale image collected by a camera is collected; edge features of the grayscale image are detected, and the thickness of the mountain fire behaviors is acquired, wherein the edge features include directions and amplitudes and give a warning to the state standard of the current fire behavior; when the mountain fire behaviors are acquired at four continuous time points, a gray prediction model about the mountain fire behaviors is established; delta value compensation is adopted to correct the gray prediction model, and a prediction curve of the mountain fire behaviors is obtained; and the state standards of the mountain fire behaviors are predicted in real time according to the prediction curve. Through the early-warning method, mountain fire image collection and the gray prediction model are comprehensively utilized and combined to perform analysis and early warning on a transmission line mountain fire, and reliable prediction is made.