Abstract: Since the reform and opening up, the rapid transformation of Chinese society has almost become an important structure to identify the current Chinese society. Metropolis as a sign bearing modernization，in the intertwined role of various new and old systems, produce a large of low-income population, which gradually form a "underclass society" in sociological sense. For in-depth understanding of underclass society in metropolis，we selected low-income population in Shanghai as respondents, trying to use a different perspective to analyze reasons of formation and impact of the underclass society in metropolis .

Key words: metropolis, structural differentiation, underclass society, underclass group, impact