A major function of institutions import has been to formalize and immobilize the formerly spontaneous social stratification,and to label different classes with clear identifiable character,resulting in an obvious class gradient.This research identifies another social stratification mechanism by looking at differentiation of the migrant population resulting from the residence registration system in the big cities.As far as floating population is concerned,the income gap among different stratum is not very large;however social security,employment,education opportunity and upward mobility vary considerably across different sub-stratums.Re-stratification of stratums arises from interactive effects among institutional factors,human capital,unit and social relations.Public policies are not neutral,and are of inequality nature with selection or discrimination.