Abstract: Based on the hydrophilic testing system innovatively designed by China University of Mining and Technology(Beijing)，test methods and procedures were proposed.The water absorption tests under water pressure and without water pressure on rock samples from Pingzhuang Coal Mine were carried out，and the curves of water absorption processes were formed.The variation principles of the soakage and water absorption velocity under the different water pressure were obtained.Water absorption processes were divided into two stages：absorption slowdown first and constant speed later.The water absorption function curves were fitted.The different microstructure before and after water absorption were compared with SEM，X-ray diffraction experiments，and the results demonstrate that mixed-layer of illite and smectite in the selected rock samples changed obviously before and after water absorption，which was the major factor affecting the laws of absorption of the rock.

Key words: Sandy mudstone, Hydrophilic mechanism, Absorbed water, Test of water absorption without water pressure