The current ethno-linguistic landscape of mainland Southeast Asia is a result of the spread of Tai speakers from southern China. This study examines Chinese loanwords in Proto-Southwestern Tai, the hypothetical ancestor of all modern Southwestern Tai varieties and proposes a dating of the spread of Southwestern Tai languages. By comparing the reconstructed Proto-Southwestern Tai forms with corresponding Chinese forms, four layers of Chinese loanwords existed in Proto-Southwestern Tai, namely Pre-Later Han, Later Han Chinese, Early Middle Chinese, and Late Middle Chinese layers. These layers indicate that Proto-Southwestern Tai was in contact with Chinese at least until the Tang era. In collaboration with non-linguistic evidence, this paper therefore proposes that Southwestern Tai languages began to spread southward sometime during the eighth and the tenth centuries CE.