This paper probes the elements of theoretical necessity, starting with the word ruo (若) in Duan Yucai’s Shuo Wen Jie Zi Zhu (段玉裁《說文解字註》). From the prospective of theoretical methods of “Ruo (若) ,” the theoretical necessity of “Ruo (若) ,” the exegesis functions of “Ruo (若) ” and different intonations with different modal particles, the author conducted multi-level and multi-dimensional analysis on “Ruo (若) ” in Duan Yucai’s Shuo Wen Jie Zi Zhu (段玉裁《說文解 字註》) so as to explore the concepts of theoretical necessity from Duan Yucai.