Soil is a granular medium and its strength and deformation characteristics show a strong particle size effect. On the basis of their connection characteristics and the ratio of micro-forces and gravity, soil particles are decomposed into matrix particles and reinforcement particles. In addition, a soil cell element that can describe the internal material information and particle characteristics of soil is constructed; and a soil cell element model that can characterise the particle size effect of soil is proposed. A series of direct shear tests on saturated and remoulded soil is conducted to study the particle size effect of soil and to quantitatively determine the strain gradient and intrinsic length scale of soil. The results show that the shear yield stress of soil increases with an increase in the volume fraction of the reinforcement particles and effective strain gradient, and has an approximately linear dependence on the volume fraction of the reinforcement particles. Moreover, the relationship between the shear yield stress of soil and the effective strain gradient can be presented as a parabolic function. The size of the reinforcement particles has little influence on the shear yield stress of soil. The experimental data can be well fitted to the cell element model.