Soil is a complex granular medium and its strength and deformation characteristics behave strong particle size effect. On the basis of the physical effects of cohesion and friction generated by the interactions between soil particles at different scales but not just the geometric dimensioning, a soil cell element that can describe the internal material information and particle characteristics of soil is constructed by dividing particle size into different scales to investigate the influence of soil particles at different scales on the macro-scale mechanical properties of soil. According to the mechanical responses of soil at various scales, the notion of ratio between micro-forces and gravity is introduced; and a multiscale and hierarchical soil cell element model is proposed to interpret the mechanism of the particle size effect of mechanical properties of soil. Therefore, the microscopic soil mechanics is promoted from qualitative analysis to quantitative calculation. A series of unconsolidated and undrained triaxial compression tests on saturated, remoulded soil are designed to study the particle size effect of soil and to quantitatively determine the strain gradient and intrinsic length scale of soil. The experimental results and theoretical analysis show that the particle size effect of the strength and deformation of soil is increased with raising volume fraction and decreasing size of the reinforcement particles, which suggests strong particle size effect. The theoretical prediction of size effect is in good agreement with that of the test result.