To establish a classification on the basis of genetic linguistics requires two indispensible conditions: common retention, including cognate lexical retention and cognate morphological retention, and common innovation in a language group. The first condition is primary, and is the motivation for classification and the evidence of a genetic relationship. The second condition is secondary and supportive, and is the feature,approach and sign of a language group. This article focuses on common innovations in the Sino-Tibetan language family. Tone, voiceless aspirated stops, affricates, classifiers, noun prefixes for kinship and overlap are six common innovations that are proposed for defining the Sino-Tibetan language family.