Activity of Yellow River sediment was stimulated by phosphoric acid. Lime soil and sand soil were prepared with activated sediment and 20% of lime and river sand through press forming. Its mechanical properties and microstructures were studied; the activation effect and mechanism of phosphoric acid on the Yellow River sediment were discussed. The results show that the activation effect reaches the peak when the use level of phosphoric acid is 4%(by mass). At this use level, the compressive strength and softening coefficient of lime soil are 19.3MPa and 0.96, respectively, increasing 150% and 50% compared with the original sediment. Filamentous calcium phosphate crystal is produced by the reaction of phosphoric acid with calcium carbonate on the surface of clay particles, and —OH group content in the sediment is increased through activation of phosphoric acid. SEM and EDS analysis show that sand is wrapped around calcium hydroxide and calcium phosphate crystal. The micro morphology of lime soil activated by phosphoric acid is similar to that of ordinary concrete.