Theravada Buddhism has tenaciously survived in South and Southeast Asia society for more than two thousand years, casted the national states and people’s life here, and made indispensable contributions to the whole Buddhism including Northern Buddhism and Tibetan Buddhism. The everlasting powerful source of Buddhism is within itself, and the adaptability from its interaction with the social environment. Buddhism depends on social economic and political development, but also provides a spiritual guide to society. For this reason, Buddha’s preach has stayed in the world for such a long time, advanced with the times, and eternally renewed in the long run. Since entering the modern society, Theravada Buddhist countries have decisively karmic encounter with the western world, and make its own development integrated in global modernization process. This article discusses issues from three aspects as the important theoretical and the practical value of Theravada Buddhism, the social and political functions of Theravada Buddhism and the adaptation of Theravada Buddhism to the modern trend.