Shaft assemblies serve as critical components in numerous power systems, whose stable operation directly dictates production continuity and overall system safety, rendering real-time monitoring imperative. However, existing measurement tec[ ]hniques often fail to perform effective in-situ monitoring without interfering with the operation of large-scale system components such as rotor shafts. To address this limitation, this paper proposes a novel measurement approach for rotating shaft equipment based on a flexible sensing acquisition system. A compact and lightweight protective fixture was designed to ensure complete adhesion of the flexible substrate to the rotating shaft. The system is powered by a thin-film lithium battery, and vibration signals are captured via a triaxial accelerometer, enabling real-time in-situ vibration monitoring of the shaft without introducing additional load excitation. A method for identifying monitoring data under unbalance and rubbing conditions was developed, offering a technical framework for the monitoring of large shaft power systems such as wheel axles. Experimental results demonstrate that for unbalance faults, the radial fundamental frequency amplitude increases significantly with the introduction of unbalance excitation. In cases of rubbing faults, the frequency spectrum in the X-axis direction exhibits notably elevated higher-order harmonic multiples, and the X-axis root mean square (RMS) value is shown to be more sensitive for detecting rubbing damage.