The invention discloses a three-dimensional investigation method for a dangerous falling rock based on an air-borne laser radar. Original laser point cloud data and original video image data which cover an engineering zone and the initial exterior orientation elements of the original video image data are collected and processed to generate a DEM (Digital Elevation Model), a DOM (Digital Orthophoto Map) and a color laser point cloud. An index database is generated, the dangerous falling rock is recognized by information in the index database, the information factors of the dangerous falling rock are extracted to classify the hazard rate of the dangerous falling rock and provide a treatment strategy, and cross section lines required by a treatment design are extracted. The high-precision color laser point cloud, the high-precision DEM and the high-precision DOM are established by the advanced space-to-land observation technology based on the air-borne laser radar, the information extraction, quantitative analysis and risk evaluation of the dangerous falling rock are realized, and a novel method for surveying mountain hazards based on the air-borne laser radar technology is created.