Since the Reform and Opening Up, small towns have been highly expected in the process of promoting urbanization in China. But, the outcome is still worthy of in-depth discussion. Based on the survey data of four typical small towns in Guangxi, the paper analyzes the social reproduction process of small towns in underdeveloped areas from the perspective of "five productions" of society. It shows that the weakness of production capacity is the key factors for local residents’ migration, the alienation of local spiritual production process weakens the geographical identity of small town residents, and human, social relations, natural relations have not achieved goal of sustainable reproduction. The incomplete reproduction chain has led to the urbanization in different places and semi-urbanization of small towns in underdeveloped areas. In the process of urbanization, the choice of residents in small towns is more passive selection under realistic conditions. It is difficult to play the same role as large, medium and small cities as a space entity. Local development should be coordinated in a wider area. The relationship between "five productions" should be actively coordinated based on comparative advantages, and small towns should play a greater role in promoting new urbanization.