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 viscoelatic pipes was further developed by combining a quasi-two-deimensional friction model with a two-element Kelvin-Voigt (K-V) viscoelastic constitutive model. Based on a total 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.