In previous research concerning the Ancient Vietnamese phonology using Chu Nom materials, only a few features, such as initial consonant clusters, have been considered. This study aims to reconstruct the phonological system of Ancient Vietnamese initials, using the Sino-Vietnamese version of the Buddhist sutra Phat Thuyet Dai Bao Phu Mau An Trong Kinh 佛説大報父母恩重經, as well as the Proto Viet-Muong phonemes established by Ferlus (2009). The Chu Nom characters examined in this study allow us to consider topics such as spirantization and the voicing of medial consonants - possible thanks to the existence of a number of disyllabic words contained therein. Linguistic analysis of such texts does and will play an important role in filling the missing link between Proto Viet-Muong and Middle Vietnamese (17C).