Abstract: In order to reveal the microstructure multi-phase coupling mechanism and the dynamic deformation law of the silt lightweight soil under cyclic loading, the mesoscopic multi-phase deformation mechanism and the relationship between macroscopic and mesoscopic deformation characteristics of lightweight silt soil under cyclic loading are studied by using a new indoor consolidation test and the ABAQUS finite element numerical simulation methods. The results show that the EPS content and the particle size have a significant effect on the meso-deformation characteristics of silt lightweight soil. When the EPS (expanded polystyrene) content differs by 2.0%, the maximum volumetric deformation of the silt lightweight soils differs by 4.1%. Under the same number of cycles, the relations between the deformation and the stress and strain magnitude of the EPS particles at the upper, middle and lower points in the sample are different. The deformation of the EPS particles at the upper point is the largest. The mesoscopic deformation relationship of each phase of the silt lightweight soil in different cycle periods is analyzed by a quantitative calculation. The results indicate that the deformation of the EPS particles is dominant in the whole deformation process of the silt lightweight soil, and the amount is about 60%\sim80% on average.