Abstract: Based on the understanding that the expansibility of rock and soil mass is a function of water content, the concept of water absorption expansion stress coefficient of rock and soil mass is proposed. Starting from the relationship between the expansion stress and the water content, a test device for water absorption expansion stress coefficient is developed. According to the relationship curve between the expansion stress and the increase of water content obtained by the test and the calculated water absorption expansion stress coefficient of rock and soil mass, the change law of the water absorption expansion in different states and types is studied in depth. The experimental research results show that the error of final water content of the soil sample is about 1.0%, and the water absorption expansion stress coefficient of different type of rock and soil mass can obtained effectively. Due to the increasing hydration degree of clay particles, the expansion stress of each rock and soil mass increases with the increase of water content, and the expansion stress is linearly related to the increase of water content. The water absorption expansion stress coefficient of the Chenggong (Yunnan Province) expansive soil, Chengdu expansive soil, and Chengdu silty clay are 92.2 (kPa/%), 42.6 (kPa/%) and 0.9 (kPa/%) respectively under the initial water content of 15.0%. Because of difference of clay content in different types of rock and soil mass, there is a large difference in the water absorption expansion stress coefficient of different types of rock and soil mass. The water absorption expansion stress coefficient can effectively distinguish the expansibility of rock and soil mass, and it can be used to classify the expansibility of rock and soil mass.