The water seepage damage in the chest-abdomen area of the Leshan Giant Buddha is significant, which causes the sandstone in the body of the Buddha to corrode and crack, makes the repair materials on the surface of the Buddha hollow and desquamate, and leads to the growth of biological damage. In this study, continuous monitoring and sampling of the water seepage in the chest-abdomen area of the Leshan Giant Buddha and the atmospheric precipitation were performed. The water seepage in the chest-abdomen area of the Buddha is primarily controlled by atmospheric precipitation and evaporation. The hysteresis effect of water seepage reflects that the bedrock fissure on the left is relatively developed with significant water transmissibility. During the atmospheric precipitation monitoring period, the average pH of atmospheric precipitation was 8.13, i.e., weakly alkaline. After 2009, the annual average pH of atmospheric precipitation for the Buddha was greater than 5.6 and increased year by year. The water seepage of the Buddha contained a large amount of ions. The chemistry type of water seepage in the chest-abdomen ares is calcium carbonate (CaCO3) and calcium sulfate (CaSO4). Rock weathering contributes more than 90% of the ions in the Buddha water seepage. The microscopic analysis of the rock shows that the main components of the rock are CaCO3 and SiO2, and there is obvious layered morphology, is easy to be dissolved in the presence of air and water. The composition of water chemical ions indicates that H2CO3 and H2SO4 are both involved in the process of the weathering of the sandstone.