To better understand the water absorption (WA) characteristics and changes in mechanical properties caused by water absorption of tuff, carboniferous sandstone and argillaceous sandstone, this paper conducted the X-ray diffraction, Scanning Electron Microscope (SEM) and Mercury Intrusion Porosimetry(MIP) tests to obtain their mineralogical composition, micro-structure feature and pore size distribution features. A soft-rock hydrophilic experiment system was used to study the WA characteristics of rock samples, and the strengths of rock samples before and after absorbing water were tested. The results show that the WA process of three rocks includes the non-linear stage and linear stage, and the functions of the WA characteristic curves are all negative exponential function. The WA value has positive correlation with the clay mineral content and the amount of the effective holes. The reduced strength of the rocks shows negative correlation with WA value and clay mineral content, moreover the little WA value can contribute bigger effect on the mechanical properties of rocks.