Black shales are a type of sedimentary rocks that are characterized by large variation in chemical compositions both spatially and temporally. Therefore, the chemical indices, such as CIA(MIA), CIW, CIX, PIA, STI, R, WIP, V, and W, which are based on relatively homogeneous geochemical compositions of different igneous rocks, can not be used to identify the weathering degrees of black shale because they can not differ chemical compositions of parent rocks from that of weathered black shales and have low sensitivity and other problems. Therefore, it is necessary to establish a new chemical index to identify the weathering degree of black shale. In combination with the one way ANOVA analysis and the multivariate discriminant analysis methods, this study conducted major elements analysis for the samples from fresh and weathered black shales of Lower Cambrian in central Hunan to define the differences of their chemical components. Establishment of major element discriminant method can further define chemical weathering index of weathering degree of black shales. It is shown that different major elements have different degree of influence on the chemical difference between the fresh and weathered black shales, and such influencing degree decreases with an order of TiO2>Al2O3>Fe2O3>LOI>MnO>CaO>SiO2>MgO>K2O>Na2O>P2O5. Chemical weathering index for identifying weathering degree of black shales (WB) can be established as follows: WB=288×ln(SiO2)+1667× ln(TiO2)+1052×ln(Al2O3)+562×ln(Fe2O3)-201×ln(MgO)+410×ln(CaO)-424×ln(K2O)-506×ln(Na2O)+507×ln(LOI)-15813. This new chemical index can overcome the weakness of all the existed chemical indices, and can be used to identify the weathering degree of black shale.