The theory of water evaporation in soil and the soil-water characteristic curve are the core of unsaturated soil research, and have very important guiding significance for engineering practice. This paper uses the theoretical summary method to summarize the theory of water evaporation in soil and the related theory of soil-water characteristic curve. The results show that the water evaporation in the soil is the result of the combined action of internal factors and external factors. Matrix suction is the core of the soil-water characteristic curve. There are many factors that affect the soil-water characteristic curve. The mineral composition and pore structure of soil are the two most essential influencing factors, and other influencing factors mainly play their role through these two factors.