Underwater compressed air energy storage (UWCAES) is vital for balancing power supply-demand fluctuations but faces challenges of instantaneous overpressure and pressure oscillations in flexible balloons during deepsea operation and dynamic charging/discharging. This paper proposes a fuzzy PID (proportional integral derivative) - based method to suppress these pressure fluctuations. First, a dynamic pressure transmission model incorporating underwater environmental parameters is established for the balloon. Then, a fuzzy PID control algorithm is developed, utilizing the pressure error and its rate of change as inputs. This algorithm constructs membership functions and a fuzzy rule base to dynamically adjust PID parameters in real-time, optimizing the valve opening adjustment rate. Finally, case studies confirm algorithm robustness under dynamic conditions like vortex-induced shock. By achieving a 26.7% reduction in pressure standard deviation (to 30.8 kPa) over PID control, the proposed strategy effectively mitigates overpressure and fluctuations, advancing the deployment of underwater flexible compressed air energy storage.