非线性教材中对于各向同性介质，特别是各向同性手性介质的三阶极化率张量元的讲解不够详细、直观. 本文提出了一种基于级联二阶效应的分析方法，以确定各向同性介质中的三阶极化率张量元是否非零. 相比基于群论的对称操作法，级联二阶效应方法能够更快速地进行判断，并给出直观的理解. 本文还将所有非零的三阶极化统一于一个矢量表达式，进而分析了各向同性介质中三阶效应产生对输入场传播方向的限定条件，并和二阶效应进行了对比. 

In textbooks, the third-order susceptibility tensor elements in isotropic media are not detailed and intuitive enough to be explained, especially in isotropic chiral media. A analysis method based on cascaded second-order effect is proposed in this paper, to determine whether third-order susceptibility tensor elements in isotropic media are zero. Compared with the symmetry operation method based on group theory, this method can make judgement more quickly, and give intuitive understanding. The limiting conditions of propagation directions of input fields required by the generation of third-order effects in isotropic media are analyzed, and a comparison with the second-order effects is made, where the key is to unify all non-zero third-order polarizations into a vector expression.