This article presents a terahertz (THz) fast line-scanning imaging system with three-dimensional (3-D) focus-steering capability operating at 0.1 THz. The system comprises a 3-D printed rotating multi-prism plate and a dual-device structure consisting of a negative ridge pyramid and a column ridge pyramid. The simulation and experimental results demonstrate that the system generates a sheet-shaped diffraction-free beam with a projection distance of approximately 175 mm and a diffraction-free distance of approximately 200 mm. Moreover, the system maintains a resolution greater than 4 mm within the diffraction-free range. Furthermore, the proposed THz lens-less line-scanning imaging system enables 3-D scanning imaging within a set range of ±22°. The proposed approach can be extended to cover other frequencies within the THz range by appropriately adjusting the parameters. The system has the advantages of long working distance and long depth of field, making it a very attractive candidate for low-cost, easy-fabrication, and easy-adjustment solutions for the next generation of THz fast detection and imaging technology.