Abstract: Tortuosity is an important parameter in studying the permeability of soil, which reflects the flow path of fluid in soil particles. In literatures, the tortuosity is considered to be solely related to the porosity of soil. However, the expressions for the tortuosity vary greatly in the previous studies. It may be caused by some other unconsidered factors. The physical model for fluid passing through square particles in laminar flow state is established in this study. The results show that the proposed model agrees well with the numerical simulation results and the experimental results in the literatures. Finally, the influences of particle arrangement and distribution on the tortuosity are analyzed, and the value region of the tortuosity is determined. It is concluded that the tortuosity of particles relates to the porosity of soil and the distribution of particles.