Sino-Vietnamese is one of the important sources for reconstructing Chinese phonological history. The phenomenon of some lai (來) initial words pronounced as s- in Sino-Vietnamese is often considered as the evidence of consonant clusters in Old Chinese. This study reanalyzed the Lai initial words pronounced as s- in Sino-Vietnamese according to the phonological histories of both Chinese and Vietnamese, from the aspect of language contact. Examination on Austro-Asiatic cognates and analysis of phonological strata were proposed in order to check the validity of their origin. It is found that not all of these s- lai initial words are from the same origin but from multiple sources. Some of them were borrowed from Old Chinese. Some of them were borrowed from late southern Chinese dialects. And some of them are not from Chinese origin and cannot be considered as the evidence for reconstructing Old Chinese.