Abstract: This paper is based on types and characteristics of clay minerals forming the argillanceous cement of the sandstone at Longyou Grottoes in Zhejiang Province. The water samples were further collected separately from (1) water seepage in accumulation on the ground level within the caves, (2) water seepage infiltrated through cracks on the ceiling of caves, (3) surface water outside the caves, as well as (4) the water from Qujiang River near Longyou Grottoes. They were investigated and analysed using chemical analysis methods. The results show that the mineral components and structure of the sandstone depended on the water quality. The interaction between water and sandstone was the main cause of different quality of the water seepage. Sandstone samples from the caves were prepared and then immersed separately in the above water samples and distilled water. Each of the immersion water was sampled and analysed at regular intervals. The mechanism of the chemical interaction between water and sandstone could be found. The chemical interaction results indicate that the main impact of the water seepage infiltrated after atmospheric precipitation on Longyou Grottoes is a series of reactions such as hydration, dissolution and oxidation which take place during the infiltration process. Hence, the mineral components and the structure of the sandstone are changed. The quality of the water seepage is implicated directly in these reactions. Therefore, the clay minerals and the water seepage can interact on each other. Such characteristics of the clay minerals in sandstone with water are the main reason causing weathering of the sandstone.