The murals and statues of the Mogao Grottoes are unique and fragile. The dry environment is one of the key factors guaranteeing the preservation of the site over a long time. Rainfall can cause a lot of damage. Based on 25 years of meteorological monitoring data, it is found that the average annual precipitation at the Mogao Grottoes is 39 mm, most being atmospheric rainfall. The precipitation was not evenly distributed, May to August were the periods that received the most rain.For the most part, there were slight and small amounts of rain, counting for 76% of the total rainfall. The heavy rains and storms accounted only 1.5% of the total, but they resulted in regional floods. A rainfall simulation experiment was done. The experiments show that when the average rainfall intensity is 0.75 mm/min and lasts for 160 minutes, the rainfall wetting front reaches a depth of 80 cm and then maintained and equilibrium. However, at adepth of 2m to 3m, the level of water saturation can increase to about 60%. This high saturation can drive the soluble salts enrich on the base layer of murals, causing damages to the wall paintings. The Gobi runoff coefficient in the roof of Mogao grottoes is 0.016; It is inferred that the runoff capacity is very low. There would not be a regional flood if only the Mogao Grottoes area had a storm. Flooding is mainly caused by a confluence of rainfall in the Gobi area around Mogao Grottoes and in the Yema mountain region.