The social security is a very old problem of mankind society. However, the word of "social security" appeared firstly for no enough one hundred years. To see through the history of mankind society, it is not difficult to discover that Industrial Revolution of 18 century was a turning point on the development history of social security system. On the agricultural era before that, it was basic mode of traditional social security to provide disaster relief and to relieve poor people. On the industrial period after that, the social insurance was gradually becoming essential mode of modem social security.China is an ancient nation. In very long agricultural age, the plain thought about social security is of ancient origin and long development, and there are lots of historical records about practice activities of traditional social security, for instance, to provide disaster relief, to relieve poor people and so on. Since modern times, along with the beginning and development of industrialization, Chinese society was stepping into a changing-pattern period from traditional agricultural society to modern industrial society. And the mode of social security was also transforming from tradition to modern. Legitimization is the only way that the traditional mode of social security transformed into modern, the social security legislation of Modern China commences an undertaking in this case.Social security legislation would produce only when some definite historical conditions were all ready. The first half of 20 century, regarded the Republic of China as historical boundary marker, is a very short time in the few thousands process of China history, but it is undoubtedly a historical period while all aspects of society were changing most intense and most breathtaking. In this period, the initial industrialization pushed traditional agricultural society forward modern industrial society. The other factors, such as the rise of workers class, the change of natural surroundings because of calamity, the vagrant problem because of large-scale population flow rapidly, further intensified social vicissitudes. Social transformation and vicissitudes easily led to disproportion of social structure and many social problems. However, the single social relief mode could not deal with new social problems by industrialization. Under such historical background, it had become an inexorable trend of social development to establish the social security system suited the needs of the industrial era through nation's lawmaking. Therefore, this article analyzes social cause and historical background of Modern China's social security legislation in the first chapter.In Minguo(1912-1949) era, because that the society was changing and the industrialization was unprosperous, the social security mode of agricultural period still seized an important position. The traditional social security thought and practice not only brought practical effect into full play, but also became an important thought source that the traditional social security mode turned to the legitimization. On the other hand, Modern China industrialization had after all initiated, it had become an objective trend of social development that the social security mode turned from tradition to modern. The entrance of Western social security theory offered theoretic base and internationalreference for modern transformation of China social security mode. ILO's conventions and recommendations, about international labour as well as social security, established the international standard for Modern China's social security legislation. At the same time, the internal academic circles' learned research on social security problems, such as social relief, social insurance, social welfare and so on, also provided theoretic reference for social security legislation. This article analyzes the thought source and the theoretic base of Modern China social security legislation from above-mentioned four aspects.Social security legislation is generally named social legislation. It is interdependent with social polic