<p>Deep underground engineering often involves hard rock subjected to high stress, complex stress paths, and long-term loading, under which pronounced nonlinear and time-dependent responses may develop. To support laboratory investigations under such conditions, this study develops a true triaxial testing apparatus for hard rock. The apparatus adopts a three rigid and one flexible boundary configuration and enables independent servo control in three directions, high-stress load holding, and prescribed complex stress paths, including principal stress rotation, loading–unloading transitions, and long-term constant or stepwise loading. A series of benchmark verification tests was designed and conducted to validate the controllability and reliability of the apparatus, focusing on servo stability, long-term load-holding performance, repeatability, and synchronization of multi-source deformation measurements. Typical stress–strain responses and macroscopic failure observations obtained under representative stress paths are reported as verification evidence of the apparatus performance in critical operating scenarios. The results demonstrate that the apparatus can stably impose and switch the prescribed stress paths and can meet the requirements for complex loading and long-term loading tests under high-stress conditions. The outcomes are presented for apparatus verification and methodological demonstration, providing a validated experimental platform for future mechanism-oriented studies on deep rock failure and time-dependent behavior.</p>