To explore the economically suitable soda residue foundation soil with excellent bearing performance， 24 kinds of composite soda residue soil mixed with fly ash and soft clay were designed. Taking both unconfined compressive strength and economic factor into consideration， the optimal proportion was determined through fuzzy evaluation method. Splitting tensile test， static triaxial test and creep test were carried out for composite soda residue soil with optimal proportion. The results show that both soft clay and fly ash have obvious reinforcement effect on alkali slag. Addition of soft clay can reduce the amount of fly ash and has good economical applicability. The ratio of splitting tensile strength of composite soda residue soil to its unconfined compressive strength is 0.15. The consolidation confining pressure has the greatest influence on the undrained strength of the composite soda residue soil， followed by the over-consolidation ratio and the consolidation ratio is the least. When the upper load does not reach the long-term strength， the creep characteristics are relatively stable and the value is extremely small.