Abstract: Mo Sou (Collected Essays on Calligraphy) of the Tang Dynasty has been misinterpreted all the time: some claim it is vulgar, others believe it was written by the great calligraphy master Wang Xizhi. Taking advantage of the research findings of modern folklore, this thesis probes into the issues such as the author of Mo Sou, its completion process, the features of its writing technique teachings and its social background and finds out that Mo Sou was not completed in a short time by a single author. In fact, it was added, improved and eventually finalized to be its extant version during the popular spreading in the Tang Dynasty. Its writing technique teachings are as same as those of folk arts and crafts and characterized by being systemic, confidential and mysterious. The appearance of Mo Sou was closely related to the demands for writing in classic copying, imperial examination system and political administration in the Tang Dynasty.