The invention discloses a cable tunnel routing inspection flight method for an unmanned aerial vehicle. Along-line flight is involved and comprises the steps that a highlight color with an obvious contrast ratio to a surrounding environment of a cable tunnel is selected, and a guide line with the color is laid in the center of a flight area between a starting point and a terminal point of the tunnel which needs routing inspection; then, a camera is adopted for obtaining a front real-time image, an interest area is cut, and the real-time performance of the system is improved; then, color extraction is conducted, binaryzation is conducted, an image after binaryzation is subjected to morphological filtering, and noise is removed for guide line extraction; finally, the image after binaryzationis scanned, a least square method is used for fitting a straight line, and accordingly the position and posture of next-moment flight of the unmanned aerial vehicle are solved. According to the cabletunnel routing inspection flight method for the unmanned aerial vehicle, a rail does not need to be laid, the guide line with the highlight color only needs to be arranged, so that the routing inspection cost is low, and the problem can be solved that since equipment is placed in a dark and damp tunnel environment for a long time, the equipment is easily damaged.