The invention provides a power transmission line fog level recognition method and system based on images. The method includes the steps that a plurality of training images of a power transmission line in the weather of sunshine, light fog, fog, heavy fog, smog and thick smog are collected; image classes corresponding to the sunshine, the light fog, the fog, the heavy fog, the smog and the thick smog are established according to the training images; characteristics of the image classes corresponding to the sunshine, the light fog, the fog, the heavy fog, the smog and the thick smog are extracted respectively; the characteristics of the image classes are used as input data of a naive bayes classifier to be trained so as to obtain a fog level recognition template base; an image to be recognized of the power transmission line is collected; characteristics corresponding to the image to be recognized are extracted; recognition is conducted on the characteristics corresponding to the image to be recognized according to the fog level recognition template base to obtain a recognition result; the recognition result of the image to be recognized is output. The fog levels are divided into the light fog, the fog, the heavy fog, the smog and the thick smog according to horizontal visibility distances, and classification recognition of the fog levels is achieved.