Yen Fu's social thought, which centered on the ideas of struggle for existence and natural selection, had a great influence on modem China. His social thought integrated the evolutionary theories of Darwin, Spencer, Huxley and his own comments and interpretations of them. With a highly sophisticated contents and intellectual background and contains contending ideas toward the role of natural selection in human society, his evolutionary thought is very difficult to be clarified. This article tries to use a comparative approach to analyze Yen Fu's evolutionary ideas, so as to better understand the meaning and characteristic of his theory, the debate involved, the difference between the original theories and his translation, and his modification and synthesis of Western ideas.