This paper reconstructs the Khitan vowel system by analysing materials concerning the Khitan Small Script. First, the approximate phonetic values of the graphemes were determined by systematically comparing Khitan transcriptions of Chinese words with their original Chinese sounds and by analysing Khitan rhymes. Next, an exhaustive survey of two adjacent graphemes in a corpus elucidated the script’s spelling rule and thus were the accurate phonetic values determined. Finally, a comparative study based on the reconstructed values established regular vowel correspondences between Khitan and Mongolian. In conclusion, the author presents twelve Khitan vowel phonemes that are distinguished by four vocalic features.