This study established the relative chronology of sound changes in the Lanna Tai language from Proto-Southwestern Tai to Modern Lanna Tai dialects. The 16th-Century Lanna Tai phonology, as documented in the Sino-Lanna Tai Manual of Translation, was compared to Proto-Southwestern Tai and Modern Lanna Tai dialects to differentiate pre-16th century and post-16th century changes. Based on this chronology, Lanna Tai can be divided into three stages: Old Lanna Tai featuring the loss of uvulars and changes in consonants; Early Modern Lanna Tai exhibiting tone split, loss of voicing contrast and vowel acquisitions; and Modern Lanna Tai dialects showing changes in consonants, acquisition of additional vowels and diphthong monophthongization in some dialects. The study focused on several sound changes, providing valuable insights into the evolution of the Lanna Tai language.