The influence of seawater flowing inside the pipeline on the cathodic polarization behavior of carbon steel pipelines was studied viaa home-made closed-loop circulation system with electrochemical tests, meanwhile, the electrochemical behavior of different locations on the inner side of the pipeline by varying polarization potentials was analyzed by computational fluid dynamics simulation. The results show that as the polarization potential varies, the current density trends at elbows and straight sections are consistent; however, the middle region of the elbow is the most susceptible to corrosion, with the outer bend requiring a significantly higher protective current density than the inner bend. The capacitive arc radius at the elbow first increases and then decreases along the flow direction, consistent with the variation in current density. Surface morphology and XPS analyses reveal that at a polarization potential of -1.2 V, a compact Mg(OH)₂ deposit forms, which enhances protection but increases current consumption due to the cathodic reaction shifting toward hydrogen evolution. Numerical simulations further demonstrate that near-wall radial velocity is highly correlated with current density and can serve as a key parameter for evaluating the influence of seawter flow on the cathodic protection effectiveness. Based on this, a relationship model between current density and radial velocity was established, providing valuable reference for the design and assessment of cathodic protection systems in flowing seawater environments.