Abstract: Niuchang ancient underground Caverns in City Longyou were built in Southern Song Dynasty and is an important part of national key cultural relics protection unit from Xiao Nanhai cave(the Longyou Grotto).They have old excavation history, rare slanting sidewall-roof structure in No.2 Cavern and plug topside structure in No.1 Cavern. The two cavern chambers get great attentions. But, serious deformation and failure happen in their roofs and slopes at the entrances of both caverns. In order to preserve the relics, this paper has carried a detailed investigation and analyzation about deformation and failure from aspect of engineering geological conditions. It shows that the long-term weathering, especially biological and thin roof composed of gentle-dip red clastic rocks are the main reasons of their deformation and failure.