Natural weathering has been the most challenging problem facing the researchers on the conservation of Yungang grottoes, one of the world cultural heritage sites in China. The weathering processe closely relates to the involvement of water and salt. In order to tackle the problems caused by water, investigations and site tests have been thoroughly carried out to locate the source of water. The main tasks in the project of waterproof protection for Yungang grottoes are established, which are seepage proof on top of the caves, protecting shelters, and research and control on the condensed water in the caves. The seepage proof task on top of the caves consists of drainage system and prevention of seepage water with emphasis on the former. The seepage proof material is suitably chosen as waterproof blanket made of bentonite. Protecting shelters play an active role in stabilizing the environment for the relics in the caves. The measurement on the condensed water on the surface of sand rock by the newly developed instrument shows that there is a significant amount of condensed water present. The damp removal machine can be used to reduce the relative humidity in the caves and furthermore preventing the formation of the condensed water on the surface.