This study investigates the corrosion behavior and mechanisms of low-alloy steels under low-temperature freezing–thawing conditions. With the increasing use of low-alloy steels in polar, cold, and marine regions, traditional corrosion research has not adequately addressed the phenomena occurring under alternating low temperatures and freezing–thawing cycles. Freezing–thawing cycles not only cause repeated freezing and melting of moisture on the steel surface but also affect the formation of corrosion product films, crack propagation, and the protective properties of the metal substrate. Addressing this research gap, the study of corrosion behavior of low-alloy steels under such conditions aims to provide a deeper understanding of the underlying mechanisms in extreme environments, thereby offering essential theoretical support for improving the reliability of steel applications in cold environments. An indoor simulation test was designed based on the annual temperature fluctuation characteristics of extreme cold environments, with controlled temperature differences and specific freezing and thawing limits. The corrosion behavior and mechanisms of low-alloy steel under freezing–thawing conditions were investigated through weight loss measurements, surface and cross-sectional morphology observation of corrosion products, compositional analysis of the corrosion products, and electrochemical testing. The results indicated that the highest corrosion rate occurred under freeze–thaw cycles at 10–0 °C, while the lowest rate was observed at −10− −20 °C. This indicates that temperature exerts a greater influence on the corrosion rate. Under all four freezing–thawing conditions, the corrosion product films exhibited cracking, providing pathways for corrosive ions to accelerate corrosion. The corrosion products primarily comprised α-FeOOH, β-FeOOH, γ-FeOOH, and Fe₃O₄, with the outer rust layer predominantly composed of γ-FeOOH and the inner rust layer mainly composed of α-FeOOH. The pitting diameter increased and then decreased with the extension of the testing cycle under all freezing–thawing conditions. Moreover, pitting exhibited a tendency for longitudinal development, reaching a maximum pitting depth of 37.9 μm. Furthermore, electrochemical processes persisted beneath the ice layer, with the lowest corrosion current density measured as 0.002 μA/cm².