The invention discloses a method for acquiring an overall 3D image of a layer-by-layer excavation project, which comprises the following steps: (1) obtaining a first-layer excavation face; (2) determining the 2D image acquisition range of the first-layer excavation face; (3) arranging mark points; (4) measuring the geodetic coordinates of the mark points; (5) continuously photographing the first-layer excavation face through an unmanned aerial vehicle to acquire a series of 2D photos of the first-layer excavation face; (6) importing the acquired photos and the coordinates of the mark points into Smart3D 3D image synthesis software to get a 3D image model of the first-layer excavation face through synthesis; (7) excavating a second layer, and obtaining a second-layer excavation face; (8) determining the 2D image acquisition range of the second-layer excavation face; (9) repeating step (3) to step (6) to get a 3D image model of the second-layer excavation face, and getting 3D image models of the excavation faces of the other layers in sequence in the same way; and (10) synthesizing the 3D image models of all the layers to form a final overall 3D image. The method of the invention greatly reduces the workload of geological personnel in the field, and improves the cataloging accuracy.