The Shuowen jiezi 說文解字, written by Xu Shen 許慎in the first century A.D., was the first etymological dictionary of Chinese characters. While countless attempts were made to annotate the Shuowen throughout the millenniums, the Qing scholar Duan Yucai's 段玉裁 contribution, Shuowen jiezi zhu, proves to be the most revolutionary. Equipped with philological tools unknown before him, Duan single-handedly overturned the entire Shuowen study. Existing researches on Duan's commentary unanimously focus on his new insights on individual etyma - losing sight, however, on what brought Duan to these original propositions in the first place. Duan was, in fact, a brilliant scholar on historical phonology, which in turn enabled him to solve the many untouchable puzzles in the Shuowen. In this article, the author attempts to investigate how Duan applied his theory of heyun 合韻 to produce an original interpretation of the Shuowen - an aspect of Duan's study which unfortunately, remains overlooked by scholars in this field.