李光地《周易折中》以「折中」治《易》，一方面折中眾說，另一方面面對眾說分歧，而「折中」於朱子，因朱子兼取象數、義理之故。李氏釋《易》重視「義例」，尤要者為「卦主」。李氏卦主理論，提出成卦之主與主卦之主，並說明如何認定成卦之主與主卦之主；此外，亦明確指出六十四卦之成卦之主，及指出部分有主卦之主的卦，而在《周易觀彖》更可見出以成卦之主釋卦爻辭的部分。本文將論述李氏如何折中歷代卦主理論，指出成卦之主與主卦之主的認定原則，並考察如何以卦主釋卦名及美辭，以歸結出李氏以卦主釋《易》之特色，作為研究李氏《易》學的可能途徑。

Li Guangdi 李光地 studied Yijing 易經 in a way similar to Zhu Xi 朱熹, who used an approach that combined both divination numbers and philosophical exegeses. In addition, Li Guangdi also emphasized the importance of the constant principles found in the book, especially that of guazhu 卦主 (a theory holding that there is one or more dominating lines in a hexagram). According to Li’s guazhu theory, each hexagram has a dominating­factor yao and a dominating-position yao. He proposes a way to identify these, and delineates the dominating-factor yaos for the 64 hexagrams, as well as identifying some hexagrams with dominating-position yaos. Li also explains the hexagrams and monograms based on their respective dominating factor. This paper shows how Li Guangdi amalgamated previous studies of guazhu to form his own theory, and explains the principles of recognizing dominating­factor yaos and dominating-position yaos. Moreover, how he used this theory to explain the names of the hexagrams and their explanations is also scrutinized. It is hoped that through understanding Li’s use of his guazhu theory to interpret Yijing we can better understand his interpretations of Yijing.