Drilling is the most direct and effective investigation method for tunnel engineering， however， for deep-buried long tunnels， traditional vertical boreholes have limitations such as less effective footage， significant challenges in equipment relocation， etc. Directional drilling technology offers a solution by allowing off-site borehole placement （e.g.， along mountainsides or valleys） and controlled drilling trajectories to investigate the geological conditions of the tunnel structure， demonstrating high economic efficiency. A branch-type directional exploration hole was successfully implemented in the water diversion project from Yangtze River to Hanjiang River. The main hole reached a depth of 630.8m， with a branch hole extending 81m， a total of 158 coring runs were conducted， recovering 166.3m of rock core， achieving a 95% core recovery rate in the target zones. Additionally， acoustic wave logging， borehole imaging， and hydrogeological tests were performed. This project established a comprehensive investigation method integrating directional drilling and in-hole testing for hydropower and water conservancy engineering. It provides a new approach for deep-buried tunnel exploration and serves as a valuable reference for applying directional drilling technology in similar projects.