Multiple pipeline cracking accidents in nuclear power plants (NPPs) are caused by the thermal cycling phenomena. In the downward horizontal (DH) pipelines, when high-temperature fluid flows through the branch pipe, it causes vortex turbulence which penetrates the branch pipe, and mixes with the low-temperature fluid. When temperature reaches a dynamic balance, the cold and hot interfaces alternately advance and retreat, potentially causing fatigue stress and leading to cracks. Since the integrity of nuclear power pipelines is crucial for the stable and safe operation of NPPs, we use ANSYS Fluent to establish a numerical model for DH pipelines. Based on large eddy simulation (LES) and by setting reasonable boundary conditions, we analyzed the impact of main pipe flow velocities on the depth of turbulent penetration. Additionally, we obtained the temperature fluctuation characteristics of the thermal cycling phenomena and explored the sensitive factors of thermal cycling. The results show that the vortex at the junction of the main and branch pipes will penetrate into the branch pipe, with its depth being affected by the flow velocity. The temperature rise in the horizontal section of the branch pipe, caused by turbulent penetration, interacts with the heat dissipation effect, forming a dynamically stable state. These findings provide fundamental data and support for optimizing pipeline design and implementing crack prevention measures in NPPs. Additionally, they offer valuable insights for the development of advanced nuclear reactors, demonstrating both engineering significance and research value.