Mg isotope fractionation in the supergene environment have been obtained many important advances in recent years, and these new knowledge supply the clues for further understanding Mg isotope geochemical cycle in the supergene environment. Mg isotope geochemical cycle in the supergene environment involves some important geological processes, such as weathering, river transportation, carbonate sedimentation and water-rock reaction. Mg isotope fractionates dramatically in weathering process. Silicate weathering products enrich in ²⁶Mg (secondary clay minerals prefer to combine with ²⁶Mg) and ²⁴Mg prefer to be into the aqueous phase. Although there is not significant Mg isotope fractionation during river transportation, Mg isotope of river water could still be affected by the supplement of external sources. Most magnesium from river water are transported into the ocean, and then the carbonate precipitation prefers to remove ²⁴Mg from ocean as carbonate minerals. Submarine low temperature waterrock reaction could transport less magnesium from ocean into the rocks during secondary mineral formation, which is associated with Mg isotope fractionation. However, most of magnesium (80%~87%) in seawater prefer to combine with the rock in high temperature waterrock reaction and recirculated hydrothermal fluid may be enriched in ²⁴Mg. In brief, magnesium from upper crust (δ²⁶Mg about -0.22‰) experiences weathering and transports by river water to ocean in the supergene environment. The magnesium in ocean could recycle into the rocks by the process of carbonate precipitation (δ²⁶Mg is less than -1‰) or reacting with submarine basalt to form secondary mineral.