<p>This paper presents a novel large-scale 3D printing apparatus for fabricating complex geological models. The 3D printing apparatus is designed based on the wet-material extrusion deposition molding process, which comprises a mechanical frame, a feeding subsystem, a transportation device, and a control subsystem. During the development of this 3D printing apparatus, some key technical difficulties were solved. 3D printing materials with good physical and mechanical similarity to natural rocks were developed. Continuous printing of large-scale complex geological models was achieved using a gantry frame and a fixed printing platform. Automated transportation of the large volume and weight geological models from the 3D printing apparatus to the loading system was completed. To verify the effectiveness of this apparatus, a 2&#xa0;m × 2&#xa0;m × 1.5&#xa0;m complex geological model with two faults was successfully printed. Based on this printed model, true triaxial geomechanical model tests were performed to simulate the step-by-step excavation process of a deep tunnel in southwestern China. The test captured the macroscopic failure process of the surrounding rock caused by high in situ stress, two faults, and excavation unloading. The model test results show good consistency with the failure characteristics of the surrounding rock in the field. This 3D printing apparatus offers a novel approach to preparing complex geological models.</p>