The invention relates to the technical field of power transmission line maintenance, in particular to a nighttime mountain fire prevention indicating device for a transmission line. The nighttime mountain fire prevention indicating device comprises a horizontally arranged supporting panel; a supporting side plate is vertically arranged on the bottom of one side surface of the supporting panel; a plurality of screw holes are formed in the supporting side plate at intervals, and mounting bolts are inserted into the screw holes; mounting nuts are threadly connected to mounting bolts; a second screw hole is formed in the center of the supporting panel, and a vertical rod is threadly connected in the second screw hole; a horziontaly arranged mounting panel is arranged on the top end of the vertical rod; an indicating lamp is arranged on a top surface of the mounting panel; and second supporting rods are vertically arranged on four corners of the top surface of the mounting panel. The invention has a good indicating and warning effect, facilitates maintenance of the relevant line an abnormal situation occurs, and facilitates practical use.