Many countries have begun to focus on the development and utilization of marine resources, involving ports, docks, oil production platforms, cross-sea bridges, large ships, and other marine engineering facilities, since beginning of the 21st century. Marine atmospheric corrosion issues encountered during construction put the safety of these marine engineering facilities in jeopardy. The Nansha Islands, which are located in the southernmost part of the South China Sea, are in a typical tropical marine atmosphere environment with no long-term corrosion data. The steel used in marine engineering is the premise of expanding marine space and exploiting marine resources, as well as the guarantee of enhancing marine national defense strength and safeguarding maritime rights and interests. Because of its poor corrosion resistance, the service life has certain limitations. As a result, studying the corrosion mechanism of carbon steel in this typical atmospheric environment after long-term exposure is crucial for engineers. The corrosion behavior of low carbon steel Q235 and weathering steel Q450NQR1 was investigated using the corrosion loss method, macroscopic morphology observation, SEM, XRD, and electrochemical and tensile tests after 34 months of exposure in the Nansha atmospheric environment. The results show that the corrosion dynamic of the two sheets of steel in the marine atmosphere of Nansha Islands can be divided into two stages. The corrosion rate of the second stage is smaller than that of the first stage. Weathering steel Q450NQR1 has demonstrated better corrosion resistance in a short exposure time. The rust layer on the skyward and field-ward sides of mild steel Q235 is thicker than that of weathering steel Q450NQR1 after 21 and 34 months of exposure, and there are more cracks in the rust layer, which could promote oxygen and chloridion diffusion to the substrate and speed up the corrosion process. The main components of corrosion products are γ-FeOOH, α-FeOOH, β-FeOOH, and Fe₃O₄, the relative contents of each product were different with the extension of exposure time. Furthermore, corrosion on the field-ward sides of the two steel sheets was worse than corrosion on the skyward sides. This is because the rust layer on the field-ward side was easily removed, resulting in a weakened resistance to corrosive medium. With the extension of exposure time, the thickness of the rust layer on the skyward side of carbon steel Q235 and weathering steel Q450NQR1 is increasing, and the tensile strength was gradually reduced. That is, Q235 and Q450NQR1 are more likely to fail owing to the thickening of the rust layer during use resulting in safety accidents.