Abstract: To determine influence of EPS (Expanded polystyrene) size on shear strength of light weight soil, the shear strength characteristics of mixed soil (cement mixed ratio of 12% and 20% and EPS volume ratio of 40%) with three kinds of EPS sizes are studied by means of direct shear tests. The results show that there are two types of shear stress-shear displacement relation curves: strain hardening and strain softening, and they are decided by cement mixed ratio, normal stress and so on. Its shear strength envelope is broken line, and it has the same shape as that of traditional structural soil. When the cement mixed ratio and EPS volume ratio are the same, with the increase of the EPS size, the wet density of light weight soil is constant, the shear strength decreases, the unit price reduces, but the strength reduction rate is far below the reduction rate of the unit price. Shear contraction or shear fracture may occur in EPS on shear planes, and it is decided by the cement mixed ratio and EPS size. Cohesion decreases with the increase of the EPS size, and internal friction angle has no obvious variation law. Shear failure processes of light weight soil can be divided into three stages: deformation coordination, progressive damage and pure friction. A shear strength model for 3 interfaces (stabilized soil-stabilized soil, stabilized soil-EPS, EPS-EPS) is brought forward. The strength reduction law of light weight soil with the increase of the EPS size is theoretically proved. According to the fact that the strength reduction rate is far below the reduction rate of unit price, EPS beads whose diameter is 3-5 mm should be used in construction, and prescription can be optimized, then the cost can be reduced appropriately.