Abstract: To address the challenges in performing fluorescent penetrant inspection (FPI) on complex internal surfaces of small-diameter long shafts in aero-engines, specialized inspection tooling was designed and fabricated, coupled with the development of tailored inspection procedures. Technical research on FPI for long shaft internal surfaces was conducted, resulting in a dedicated inspection scheme validated through systematic testing. This methodology had been successfully implemented for the internal surface inspection of a low-pressure turbine shaft in a specific aero-engine model, with potential applicability extended to analogous components.