Proto-Hlai is the reconstructed ancestor of the Hlai languages. Proto-Hlai reconstructions include those of Matisoff (1988), Thurgood (1991), Wu (2000), Ostapirat (2004), and Norquest (2007).

Peter K. Norquest (2007) reconstructs 29 basic Proto-Hlai consonants (Norquest 2007:135), while Weera Ostapirat reconstructs only 19 proto-consonants (Ostapirat 2007:145).

Norquest additionally reconstructs six onsets suggesting consonant clusters: *pl, *fj, *lj, *ɾj, *tʃʰw, *ŋ̊w. Whether these were actual consonant clusters is not clear. The clusters with a glide as a second member may have been coarticulated consonants: palatalized *fʲ, *lʲ, *ɾʲ, labialized *tʃʰʷ, *ŋ̊ʷ, while *pl may have arisen from a sesquisyllable *p-l.

Norquest also reconstructs six bisyllabic root shapes:

Pretonic syllables are later lost in all Hlai languages, but in these six combinations, they trigger in some languages conditional developments of the tonic syllable's onset consonant, allowing them to be distinguished in reconstruction.

In Ostapirat's (2004) reconstruction, Proto-Hlai forms can be both monosyllabic and disyllabic. Some disyllabic forms have medial consonants beginning with three penultimate vowels (*u-, *i-, *a-; the last of which is default). Vowels can also combine with *-i or *-u to form diphthongs. Tones (*A, *B, *C, *D) are also reconstructed.

In the table below, Proto-Hlai consonants marked as green can occur at the end of syllables.

Ostapirat (2004) reconstructs 5 vowels, which are /a, ə, i, ɨ, u/. Norquest reconstructs seven vowel qualities (Norquest 2007:238, 330).