The work aims to study the effect of dry-wet alternating seawater on corrosion rate of carbon steel. Taking seawater from Xiaomai Island in Qingdao as the test medium, carbon steel specimens and electrochemical specimens were tested for 3, 7, 14, 21 and 28 d respectively by alternating immersion equipment with 60 min as a cycle under the dry-wet alternating conditions of immersion/drying time for 10 min/50 min, 20 min/40 min and 30 min/30 min using weekly immersion equipment and then taken out. The change of corrosion rate was studied by weight loss method and electrochemical test method. The components and morphology of corrosion products and the corrosion morphology after removal of corrosion products were studied by X-ray diffraction (XRD), energy dispersion X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). The corrosion rate of carbon steel under alternating dry-wet conditions with three dry-wet ratios was 3~8 times of that under whole immersion condition, and the corrosion rate increased with the increase of dry-wet ratio, but the increase gradually became smaller. With the increase of the dry-wet ratio, the thickness of the rust formed became thinner and the content of γ-FeOOH and oxygen in the rust layer increased while the content of Fe3O4 decreased. The corrosion rate of carbon steel increases with the increase of dry-wet ratio, and the acceleration effect has a maximum value.