Abstract: In mudstone order to study the swelling law of the expansive soil during humidification process, taking the expansive mudstone soil sample from a high-speed railway foundation as an specimen, studies of the change law of non-loading expansion rate with the increase of moisture content are carried out in this paper.The experimental results indicate that the change process of the non-loading expansion rate of the expansive soil with the changes in moisture content can be divided into three stages:the rapid expansion stage, the slow expansion stage and the tend-to-be-stable stage.This study obtains the values of the moisture content in three change nodes, then utilizes the four indexes, that is, montmorillonite content, cation exchange content, free expansion rate and liquid limit, as the model factors, and further uses the main ingredient analytic method to get the Z value of the expansion characteristic, which can represent the expansion capacity of the expansive soil.This study also analyzes the correlation characteristic, which can represent the expansion capacity of the expansive soil.This study also analyzes the correlation between the moisture content of three change nodes and the Z value of the expansion characteristic, and gets the function relationships between them.In practical works, the current moisture content of the soil sample and its four indexes can be measured, then the Z value of the expansion characteristic can be obtained from the four indexes.Furthermore, the moisture content of three change nodes can be calculated through the function relationships between the moisture content of the change nodes and Z value, and finally we can assess the relationships between the current soil moisture content and the moisture content of three change nodes and judge the strength of the soil mass under the current moisture content, and provide some reference for the practical works.