In order to understand the adaptability and failure mechanism of landing gear materials for amphibious aircraft in marine environment, the failure behavior of A100 ultra-high strength steel beneath an unsteady thin electrolyte layer (TEL) and in artificial seawater (ASW) was studied via immersion test, slow strain rate tensile test and electrochemical measurements. The results show that in comparison with the test in ASW, the charge transfer resistance of A100 steel beneath TEL is significantly reduced, correspondingly, the steel corrosion is significantly promoted, and the deposition of corrosion products is more obvious. Due to the existence of TEL, the oxygen reduction process and the deposition of corrosion products was all promoted. Meanwhile, the corrosion process under the dynamic TEL was stimulated due to the presence of reduction reaction of the Fe³⁺ within the corrosion products accompanied with the efficient dissolved oxygen, which then resulted in the occurrence of obvious uniform corrosion beneath the corrosion products. Similarly, A100 steel is more sensitive to stress corrosion cracking (SCC), when the test steel is covered with TEL, because the accelerating corrosion process may lead to the decreases of the effective bearing cross-sectional area of A100 steel. At the same time, the acidification process beneath the corrosion product layer promotes the hydrogen precipitation reaction and accelerates the SCC reaction process. It follows that the significant increase of the sensitivity of SCC for A100 steel may be ascribed to both the strength loss and elongation loss during the SCC testing in TEL environment.