In this paper, the electrochemical corrosion behaviour of Q235, X65, X70, and X80 low-carbon steel was systematically studied by a variety of test techniques using natural saline soil containing 1.1% salt under laboratory conditions. The electrochemical corrosion behaviour, macro-micro corrosion morphology, and corrosion product composition of these four low-carbon steels in saline soil were studied to explore their salt corrosion resistance and reveal their corrosion mechanisms. The research results showed that oxygen absorption corrosion occurred in all four low-carbon steels in the saline soil, and the corrosion types were all localised corrosion. The corrosion process of Q235 steel was controlled by mass transfer, while the corrosion processes of X65, X70, and X80 steel were controlled by charge transfer. The corrosion rates of these four low-carbon steels in saline soil followed the order Q235 > X65 ≈ X70 > X80. Variation in elemental composition was the main reason for this difference in corrosion behaviour. Finally, microscopic test results showed that local corrosion pits were present on the surface of the steel sheet specimens, and the uniformity and compactness of the corrosion product accumulation were poor.