<p>Underwater flexible compressed air energy storage (UWFCAES) systems play a crucial role in offshore energy development and the mooring line is a core component ensuring the stability and safety of the system. However, long-term service in harsh marine environments makes the mooring line prone to fatigue damage, which poses a potential threat to the overall operational safety. To ensure the safe operation of the mooring system for UWFCAES systems, the research on dynamic tension response and time-variant fatigue reliability of mooring line is conducted. The tension time–history simulation by LS-DYNA is performed and the T-N curve is adopted to estimate the fatigue life. The time-variant reliability model is established based on the load–strength interference theory to analyze the variations of residual strength, failure probability, reliability, and reliability index over a 30-year service period. The results show that the estimated fatigue life of the mooring line reaches 8.025 × 10¹¹ cycles. During the 30-year service period, the residual strength degradation is negligible, the failure probability is maintained at the 10⁻¹¹ order of magnitude, and the reliability index is 6.3902, indicating sufficient safety margin. The established analysis framework provides theoretical and technical support for the optimal design and operation and maintenance of mooring systems for UWFCAES systems.</p>