Abstract: Viscoelastic pipeline systems are frequently subjected to periodic hydraulic transients induced by rapid valve opening/closure or pump start-up/shutdown, which may cause fatigue accumulation and consequently degrade mechanical performance and operational safety. Although the dynamic response of viscoelastic pipelines under a single water-hammer event has been widely investigated, the response mechanisms and energy-evolution laws under periodic loading remain unclear. To address this gap, an experimental transient-flow facility using high-density polyethylene (HDPE) pipes was established to perform periodic water-hammer fatigue tests. A two-dimensional transient-flow numerical model for viscoelastic pipes was further developed by combining a quasi-two-dimensional friction model with a two-element Kelvin-Voigt (K-V) viscoelastic constitutive model. Based on an integrated energy analysis method, the evolution of energy storage, energy retention, and energy dissipation in the system was quantitatively characterized for different cycles of water-hammer excitations. Results show that the first peak of pressure fluctuation is mainly governed by the initial conditions, whereas the pressure attenuation rate is highly sensitive to the fatigue-damage history. With increasing excitation cycles, the constitutive curves shift upward, and the creep compliance exhibits a nonlinear increase with both water temperature and excitation cycles, with a pronounced jump at 40 °C in the later stage of excitation (> 200 cycles). Energy analysis indicates that the total system energy and the energy retention ratio decrease in a quadratic manner with increasing excitation cycles, while the dissipation process remains relatively uniform and the dissipation ratio stays nearly constant. Fatigue damage imposes a dual effect on viscoelastic behavior—enhanced initial response but reduced sustaining capacity. This effect promotes a shift in the energy dissipation path towards viscoelastic dominance, ultimately resulting with a rapid attenuation of pressure fluctuations. Two quantitative indices, namely the total energy-loss coefficient and the viscoelastic dissipation ratio, are further proposed; both increase quadratically with excitation cycles and are capable of serving as effective indicators for assessing the fatigue state of pipelines. This study clarifies the fatigue-induced degradation mechanism of viscoelastic pipelines from an energy-evolution perspective, providing a theoretical basis for dynamic design and safe operation of pipeline systems.