As a critical component of the internal blowout prevention system in ultra-deep drilling, the reliability of the torsion spring within flapper float valves is pivotal to well control safety and drilling efficiency. The failure mechanism of fractured torsion springs retrieved from field operations was elucidated, a theoretical dynamic response model accounting was established for fluid vortex-induced vibration effects, and finite element simulation to predict fatigue life was employed. Results indicate that fatigue fracture constitutes the primary failure mode, driven by high-frequency vibrations induced by drilling fluid impact. Crucially, wear defects on the wire’s inner surface alter the stress distribution topology, causing the maximum stress concentration zone to migrate from the fixed end to the worn region on the valve plate side, thereby acting as the critical initiation site for fatigue cracks. Quantitative analysis reveals that under varying pumping parameters, the continuous fatigue life of an intact spring ranges from 55 to 420 hours. Conversely, surface wear defects precipitate a catastrophic reduction in service life by orders of magnitude, with a minimum survival time of merely 1 hour. These findings provide a theoretical basis for the design optimization of float valves and the dynamic adjustment of drilling strategies, holding significant engineering value for ensuring safe and efficient ultra-deep well operations.