The invention discloses a method for detecting arc flash based on video images. Video signals are collected through a monitoring camera set up nearby a metro track, there monitoring overhead lines; after videos are decoded, gray information of each image is obtained; whether arc flash occurs or not is judged according to gray relationships among different positions of each image and the gray relationships among the corresponding positions of the former and latter images through computer vision and image processing technologies; and if it is judged that the arc flash occurs, a series of alarm information such as an occurring site and time of the arc flash is transmitted back to a console, and related image video evidences are stored for check in a later period. According to the detection method, video processing, video monitoring and real-time alarm are integrated, the arc flash is judged and alarmed under unattended condition, and a real-time, accurate and efficient metro overhead linemonitoring system is realized.