The adaptive agents of social systems，emergence of their structures and diversity of their evolution paths make their scenario research necessary． Firstly，based on the views of complex science，this paper de-fines the profound connotation of scenarios in social systems． Then，a three-layer structure model of computa-bility of scenarios is analyzed in a social system． Thirdly，multivariate models of computable scenarios are built from three stages: actual scenarios to concept scenarios，concept scenarios to structured scenarios，and struc-tured scenarios to computer implementation． Lastly，In a typical complex system of asocial system-the Taihu lake basin，a scheme on computable and approximate scenarios of the system is proposed according to the re-search idea of the “top-down”and “bottom-up”． The impacts of different government preferences on the sce-narios evolution of the system are analyzed． This paper might provide new theoretical elements and methodolo-gy guidance for researches on social systems．