The work aims to study the effects of alloying elements Cr, Mo and Sn on the corrosion behavior of low alloy steel in simulated tropical marine atmosphere by potentiodynamic polarization, electrochemical impedance spectroscopy, chemical immersion experiment and ACM sensor. The corrosion morphology and rust layer structure were characterized by scanning electron microscope (SEM) and X-ray Diffraction (XRD), and the effect mechanism of alloying elements on corrosion resistance was discussed. The corrosion resistance of the material was the best when the content of Cr was 3wt.%. After different immersion periods, the surface of 3# (3Cr) low alloy steel primarily showed bulk particles with flocculent particles in some areas. The addition of Cr refined the particles within the rust layer, enhancing its density and inhibiting the penetration of corrosive media, thus improving corrosion resistance. Additionally, the main phases in the rust layer were α-FeOOH, γ-FeOOH and Fe3O4, and the increasing Cr content led to a decrease in Fe3O4 content within the rust layer. However, the surface of Cr-containing low alloy steel exhibited numerous deep corrosion holes in small diameter, indicating a susceptibility to localized corrosion. In order to enhance the local corrosion resistance of Cr-contained low alloy steel, three kinds of low alloy steel were prepared by incorporating 0.1wt.%Mo, 0.1wt.%Sn and 0.1wt.%Mo and 0.1wt.%Sn at the same time. Combined with the data curve of ACM sensor and the corrosion microtopography, it was found that Mo and Sn effectively reduced the occurrence of local corrosion, resulting in shallower corrosion pit of large diameter on the sample surfaces. Compared with the potentiodynamic polarization curve of 3# (3Cr) low alloy steel, the addition of Mo and Sn decreased the self-corrosion current density by 18% and 51% respectively, while the simultaneous addition of both elements reduced it by 71%. Moreover, the data obtained from the ACM sensor revealed that the corrosion inhibition rates of the 3Cr steel corrosion product film were 53.12%, 58.62%, and 73.91% for Mo, Sn, and the combined addition of Mo and Sn respectively. Furthermore, the Mo and Sn not only enhanced the corrosion resistance of the material itself but also improved the protective performance of the corrosion product film. Based on the results obtained from immersion tests of different periods, it was evident that the addition of Mo resulted in a relatively loose outer layer of the rust layer, while the addition of Sn did not exhibit any significant products on the surface of the rust layer. When both Mo and Sn are added simultaneously, the corrosion products on the steel surface are complete, the rust layer is dense, and no loss of corrosion products is observed. In the low alloy steel with Mo and Sn, the main phase components of the rust layer are α-FeOOH and γ-FeOOH. In conclusion, the addition of Mo and Sn to Cr steel can yield low alloy steel with excellent corrosion resistance.