Before the advent of debris flow, there will usually be huge noise, trench cutoff and muddy water. When a debris flow occurs, the debris flow carries a boulder impact to produce a dull sound, which is obviously different from the sounds of locomotives, wind and rain, lightning, and blasting. The flow in the trench and the water in the trench become muddy. It may be that there is landslide activity in the upstream entering the trench bed, or the debris flow has occurred and blocked the trench. This is the most obvious precursor of the upcoming debris flow. When these phenomena occur, it is necessary to evacuate to a safe zone in time according to the disaster prevention plan made in advance.