Abstract: In order to study the compaction mechanism of soft inclusion soil, and to guide the prescription design and compaction construction of light weight soil, a model for compaction density is established based on material composition and the physical essence of soil compaction. It is found that the predicted value is nearly close to the measured wet density. The absolute error is only 0.003~0.051 g/cm³, and the relative error is only 0.282%~5.267%. It is proved that the model for compaction density can predict exactly the wet density of mixed soil under different compaction conditions. The compression ratio of EPS beads and the wet density of mixed soil increase with the increase of compaction times, and the increase trend becomes slow gradually, and the void ratio of mixed soil decreases with the increase of compaction times. Under compaction conditions, the increase of wet density of light weight soil is accomplished by both the deceasing pore of soil and the plastic compression of EPS beads soft inclusion. Being similar to those of light weight soil with sand, the wet density and unconfined compressive strength of light weight soil with clay increase with the increasing compaction times. When compaction time =25, the compression ratio range of EPS beads is 6.13%~11.51%; when =94, the compression ratio range is 12.80%~14.87%. It is proved that the compaction times and energy which the standard specifies are fit for the compaction of mixed soil, and will not destroy the EPS beads. Considering that the measuring process of the parameters for the model for compaction density is complex, according to the practical engineering situation, it is supposed that the water content keeps the same during the compaction process, and the holes between soil and EPS beads after compaction are close to 0. The simplified model for compaction density is put forward, and the fact that the simplified model can be fit for the practical projects completely is proved.