The objective of this study is to analyze the formation process of ‘Cuokou-Hu’, also referred to as the ‘Phoneme [y]’, and derive a rational conclusion as to when and in which document the ‘Phoneme [y]’ was first formed. Previous studies generally recognize the separation of Ju-Yu Rhyme and Hu-Mo Rhyme categories reflected in Yunlüeyitong, during the midst of the 15th century, as substantial evidence for the formation of the ‘Phoneme [y]’. Besides this general theory, others suggest that the Dong-Zhong Rhyme category’s [yŋ], Jiang-Yang Rhyme category’s [yaŋ], Yu-Mo Rhyme category’s [y], Zhen-Wen Rhyme category’s [yn], Xian-Tian Rhyme category’s [yɛn], Che-Zhe Rhyme category’s [yɛ], Geng-Qing Rhyme category’s [yŋ(yəŋ)], etc. reflected in Zhongyuanyinyun in the early 14th century or the rhyme categories system reflected in Wufangyuanyin in the midst of the 17th century are also evidence of the formation process of the ‘Phoneme [y]’. In other words, academic circles are not in mutual consensus on when the ‘Phoneme [y]’ was actually first formed. This study develops the analyses of previous studies to suggest a new theory on when the ‘Phoneme [y]’ was formed though an in-depth phonological analysis of the separation of Ju Zimuyun and Gu Zimuyun reflected in Gujinyunhuijuyao in the late 13th century. It thoroughly reviews the Xiaoyuns as described in Ju Zimuyun and Gu Zimuyun of Gujinyunhuijuyao, using the Xiaoyuns as described in the Yu Rhyme and Mo Rhyme categories of Hongwuzhengyun as a comparative reference. Based on this analysis, this study suggests new evidence for when the ‘Phoneme [y]’ was formed, unprecedented in previous studies. Thus, the separation of Ju Zimuyun and Gu Zimuyun reflected in Gujinyunhuijuyao in the early 13th century represents the earliest documented evidence of the formation of the ‘Phoneme [y]’, also referred to as ‘Cuokou-Hu’.