Abstract: This paper aims to characterize the swelling-shrinkage and slaking features of the expansive red clay under the different initial moisture content condition. The weak expansive red clay from high-speed railway tunnel in Qingyang, Gansu Province, were tested with swelling-shrinkage and slaking tests in laboratory. Its slaking differentiation was examined through video, photographs, and images by optical and scanning electron microscope. The swelling-shrinkage, slaking changes and its fractal features of the expansive red clay were studied under different moisture contents of 3%, 8%, 13%, and 18%, respectively. Total expansion capacity has a piecewise function relationship with the initial moisture content, acquiring that the minimum swelling-shrinkage change value is located at 11.6% moisture content. The slaking time is earlier and slaking rate is increased with decreasing moisture content. The fracture development degree of expansive red clay in slaking test prensents the characteristics of rapid increase, slow increase, and stable trend with increasing immersion time. The maximum crack development degree can be up to 4.6 under the minimum moisture content condition. Maximum crack width index value is upward increased in the first half immersion time, but oscillatory decreased in rest parts of immersion time. The expansive red clay with low moisture content is more prone to macroscopic slaking than that with high moisture content, which is mainly due to the scattered agglomeration units and high percentage micro-pores in the expansive red clay under the low moisture content condition. The fractal dimension of porosity is positively correlated as a quadratic function with the initial moisture content, which resulted in the slaking more complete and slaking time shorter with lower fractal dimension.