Abstract: The mixed lightweight soil of sea silt and construction waste was made of dredged sea silt solidified by cement and lightweight insulation wall materials. Extra water was required for mixing the lightweight soil because the lightweight insulation wall materials were strong water absorption. To investigate the effect of water dryness ratio on the physical and mechanical properties of mixed lightweight soil, different proportion schemes were utilized. The theoretical optimum water requirement rate was derived and subsequently verified by laboratory dry density tests and unconfined compression strength tests. The results showed that the theoretical optimum water requirement rate was near to the practical optimum water requirement rate, the biggest unconfined compression strength and the biggest dry density decreased with the mixing amount of lightweight construction waste material increasing in the different proportions, but the unconfined compression strength and the specific strength first increased and then decreased with the water dryness ratio in the same proportions, the water dryness ratio corresponded to the maximum values of dry density. All the proportion schemes meet the demand of lightweight and high strength. The scheme with 50% of the mix ratio of sea silt to lightweight insulation wall materials gives the maximum specific strength.