Abstract: The cathodic protection of offshore platform jacket in shengli sea area was taken as the main object of study. Field surveys were conducted to assess marine biofouling conditions, while laboratory experiments examined the corrosion morphology, working potential, and activation behavior of the sacrificial anodes. Based on these findings, the key factors leading to sacrificial anode failure under the condition of marine biofouling were systematically analyzed. The results show that marine biofouling on the jacket structure reached an average thickness of 16.65 cm with a maximum of 31.7 cm, exhibiting an exceptionally high average coverage rate of 95.58%. This severe biofouling directly caused significant performance degradation of the sacrificial anodes: the actual average capacitance measured only 1 068.57 A·h/kg with a current efficiency of merely 38.8%, far below the standard requirement of ≥85%. Concurrently, the consumption rate averaged 8.21 kg/(A·a), substantially exceeding the specified limit of ≤3.50 kg/(A·a). The coverage of marine organisms inhibited the activation properties of the sacrificial anodes and intensified pitting corrosion. After 30 days of testing, the anodes covered with corrosion product layers showed an average corrosion rate of only 0.156 mm/a, maximum pit depth of 136.829 μm, and open-circuit potential of -0.89 V, while the sacrificial anodes uncovered with corrosion product layers exhibited a considerably higher average corrosion rate of 4.329 mm/a, shallower maximum pit depth of 27.185 μm, and more negative open-circuit potential of -1.09 V.