Abstract: The index used to evaluate chemical weathering degree of rock should have simple, specific relationship with the weathering degree and sufficient sensitivity to rock weathering. In addition, it should be easily determined and should not be subject to human factors. Emeishan basalt is a typical representative of mafic rocks. Besides the hydrolysis reaction that may cause leaching and enrichment of components, the oxidation of divalent iron should also be taken into account in the evaluating process for the weathering intensity of Emeishan basalt. Bivalent and trivalent iron content in fresh Emeishan basalts is stable in general. Average values of FeO and Fe₂O₃ of 910 samples are 8.45% and 5.15% respectively. They occur uniformly and randomly in pyroxene, olivine, chlorite in almond-shaped structure, magnetite, titanium magnetite and volcanic glass. The geochemistry and hydrogeochemistry of the weathered basalt, the groundwater on a basalt slope and the soaking solution of the fresh basalt show that FeO and Fe₂O₃ are more sensitive to the weathering degree of Emeishan basalt than other components, while iron is one of the most immobile major elements in basalt weathering process. Trivalent iron to divalent iron molar ratio (FF) is suitable for discriminating the weathering degree in the whole weathering process of Emeishan basalt and has higher resolution than the existing weathering indices, especially in the early weathering stage of basalt.