Characteristic features of the atmospheric transparency and its affecting factors are analyzed based on insolation data over the Tibetan Plateau and its adjacent regions from May through August of 1979. A detailed account is given of the descriptive method for analyzing the atmospheric transparency over the Plateau and of the physical implication of its various quantities.The spacial distribution of the atmospheric transparency over the Plateau and the empirical equations for calculation are also given. Finally a discussion is made of the Kastrov coefficient(Коаффечиет Кастровас С).