The purpose of this research is to clarify the corrosion mechanism of the ultralight LA141 magnesium-lithium alloy for space use, and to provide a theoretical basis for the further design of the protective coating system. Put one part of the ultralight LA141 magnesium-lithium alloy that had not been corroded and had been left in the air for a period of time with a gray-black corrosion film formed on the surface into the water, and the other part was exposed to the air, then studied its formation law of corrosion products over time, and used scanning electron microscope (SEM), polarizing microscope to observe the microscopic morphology of the corrosion products, and used energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), infrared spectroscopy to identify its composition. After the ultralight LA141 magnesium-lithium alloys were placed in water, fine bubbles of H2 escaped, and a gray-black corrosion film quickly formed on the surface. After the magnesium-lithium alloy with a corrosion film layer was placed in water, there was no obvious bubbles escape phenomenon, and the thickness of the corrosion film layer increased at a slower rate than the magnesium-lithium alloy without a corrosion film layer. The corrosion products were observed under SEM and polarizing microscope, and combined with the analysis results of EDS, XRD and infrared. The corrosion products are mainly magnesium hydroxide and lithium carbonate. Therefore, come to the following conclusions, when the LA141 magnesium-lithium alloy is placed in water, lithium metal is corroded preferentially and a large amount of hydrogen is generated. At the same time, along with the corrosion of magnesium metal, lithium hydroxide and magnesium hydroxide are generated. Lithium hydroxide is unstable in the air and will generate lithium carbonate when exposed to air. The final detected surface corrosion products are mainly lithium carbonate and magnesium hydroxide. In addition, the corrosion film can slow down the further corrosion of the magnesium-lithium alloy at the bottom to a certain extent. The results of this study have certain implications for the design and selection of materials that inhibit the corrosion of magnesium-lithium alloys.