Abstract: In order to study the influence of the swelling characteristics of soil and initial water content on the undisturbed mudstone swelling force, taking one typical area of undisturbed mudstone of the Second Double-Track Railway as the study object, three undisturbed mudstone tests have been carried out on the soils with different swelling characteristics when the initial water content is respectively at 2%, 6%, 10%, 14%, and 18%. The experimental results show that the time-history curve change of the swelling force is related to the seepage rate of the soil. The swelling force decreases with the increase of the initial water content, and the increment of the swelling force increases first and then decreases with the increase of the water content increment. The swelling force of soil is subject to content of montmorillonite in soil. When the initial water content is low, the effect of swelling mineral content on soil swelling force is significant. When the initial water content is high, the effect of swelling mineral content on soil swelling force is weakened. Based on the further analysis of the experimental data, the empirical formula of the swelling force of the soil samples under the two kinds of working conditions is matched. The experimental results are in agreement with the calculated results, which provide a certain theoretical support to the engineering construction work in swelling mudstone areas.