The effect of initial water content and initial dry density on the free swell ratio of high-speed railway mudstone was studied by the indoor free swell test. The results showed that the change rule of time-free curve of free swell ratio can be divided into three stages: rapid expansion, deceleration expansion and stable expansion. In the case of the same dry density, the swell ratio gradually increased with time, and the lower the initial water content of the sample was, the greater the swell ratio at which the water content of the expansion limit reached. The free swell ratio decreased with the increase of the initial water content. The larger the initial dry density, the faster the rate of free swell ratio decreased. The free swell ratio increased with the initial dry density, and the growth rate of free swell ratio was different at different moisture contents. The drier and denser the initial state of the sample was, the greater the expansion potential was. In addition, the relationship between the free swell ratio time-course curve and the free swell ratio-initial moisture content-initial dry density could be satisfying fitted by the semi-logarithmic function and the binary linear function, respectively; and could be better predicted the actual deformation of mudstone.