Abstract: The complexity of soil makes it difficult to name and classify it strictly, and it is almost impossible to predict its behavior accurately. The complexity of soil comes from that it is a natural product and a collection of fragmentary particles. The natural attribute of soil makes it have three-phase composition, and among them, the water in soil causes theoretical difficulties in soil mechanics, and is also the source of almost all geological disasters and engineering accidents. The so-called soil is simplified in soil mechanics teaching, and it is greatly different from the undisturbed soil in nature. In fragmentary soil, its mechanical deformation couples with physical properties, and its composition, state and structure are constantly changing in the process of stress deformation, which is the main reason why its mechanical properties are very complex. The relationship between the movement of soil particles and the various deformation characteristics of soil is revealed through DDA. The wetting tests of soil show that the plastic strain is the main factor to measure the stress-strain characteristics of soil. There is a long and close relationship between the humans and the soil, and as the carrier, material, tool and weapon of mankind, the soil is an indispensable thing for mankind. In the process of long-term life and production, people have accumulated rich experience about soil and learned a lot of lessons. Up to now, the empiricism is still the basic method in the comprehensive judgment and engineering decision-making of geotechnical engineering.