Offshore oil derricks’ service life is significantly impacted by high salt and multiple loads, which can lead to corrosion fatigue damage. However, there is no clear conclusion about the corrosion and fatigue research on the marine derrick. Therefore, tests for accelerated corrosion, tensile strength, and high-frequency fatigue were conducted on Q345B steel, which is frequently used for derricks. The morphology change, mechanical strength, and fracture law of specimens with different corrosion cycles were analyzed and explored, and the S-N curves of specimens were fitted. The results show that the specimen's surface corrosion pit number, volume, and surface roughness increase with corrosion time. Additionally, there is a good degree of resemblance between the corrosion kinetic models of the real maritime environment and the experimental corrosion, and the average speed of experimental corrosion is 15.98 times that of marine corrosion. Meanwhile, the corrosion has a small effect on the specimen's yield and tensile strength within the corrosion time of 21 days. Last, considering the change in material fatigue performance with corrosion time, the established post-corrosion fatigue S-N model of Q345B steel is more appropriate for the offshore derrick working environment than the traditional model, which is of reference significance for the evaluation of the corrosion fatigue life of offshore derrick.