Abstract: Aiming at the frequent fatigue fractures of instrument pipe welds in the small-bore piping of the Reactor Safety Injection (RSI) System of HPR1000 nuclear power units during periodic tests, a study on cause analysis and treatment was carried out. Through on-site vibration measurement, spectral analysis, and computational fluid dynamics (CFD) simulation of the flow field, it was confirmed that excessive pressure drop of the single-stage orifice plate in the low-pressure RSI pump piping induces cavitation, which is the root cause of severe pipeline vibration and fatigue fracture of the welds. A temporary vibration suppression scheme combining particle dampers and support brackets was proposed, and on-site application results showed that the vibration amplitude of the instrument pipe was reduced by approximately 42%. The research findings provide a technical reference for the vibration mitigation of small-bore piping in similar nuclear power units.