Because the stratification of argillaceous slate developed well, the anisotropy is obvious and it is easy to soften when encountering water, in the excavation, tunnel is easy to cause large deformation and collapse. In order to understand the softening mechanism of argillaceous slate in water and mechanical anisotropy and to provide reference for solving practical engineering problems, in this paper, introducing an argillaceous slate highway tunnel project in Zixi County of Jiangxi province. By preparing standard specimens in field sampling, the mineral composition analysis, water rational test and uniaxial compression test were carried out. The result shows: The main mineral composition of argillaceous slate is sericite, inclusion of a few chlorite, quartz and so on. This composition makes obvious characteristics of soft rock. The influence of hydration on the mechanical properties of argillaceous slate is large. With the increase of water content, the peak strength shows a downward trend, while the Poisson ratio shows the characteristics of slow growth. At the same time, the mechanical properties of argillaceous slate are obviously anisotropic by the direction of stratification plane. The failure modes of argillaceous slate mainly include axial tension, local shear failure and shear failure along the stratification plane.