Abstract: Deep-buried water conveyance tunnels frequently pass through complex geological settings and rugged terrains, which have long posed significant challenges for geological investigations in hydraulic engineering. Conventional exploration techniques are often constrained by limited accuracy, insufficient capability, and considerable safety risks. Directional drilling has emerged as an alternative, transforming traditional point-based investigation into a line-based approach, thereby providing a novel solution for deep-buried tunnel surveys. This study systematically reviews the progress and challenges of applying directional drilling technology in hydraulic engineering investigations. The discussion encompasses project demands for directional drilling, core-drilling techniques, integrated logging, borehole imaging, and hydrogeological testing. Representative application cases are analyzed to highlight practical effectiveness. In addition, potential development trends and future research directions are explored, with the aim of facilitating broader adoption and establishing a solid foundation for the implementation of directional drilling in hydraulic engineering investigations.