Abstract：An ancient cavern group was found in Longyou County of Zhejiang Province. It consists of 24 caverns. The finding of it evokes attentions of many scholars in the world. Field investigations were carried out；and it is found that there are many failures in the caverns. The common failures occurs at vault and pillars. There are also some cracks in the sidewalls，vaults and pillars in the integrity caverns No.1–No.5. The vault around 1–1 pillar of cavern No.1，the common wall between cavern No.1 and No.2 and the vault of cavern No.2 etc. all need to be reinforced. In order to protect the caverns，numerical analyses were also done to study the failure mechanism of the caverns. According to the field investigations and numerical analyses and on the basis of“safety first”and “rehabilitation but not change the natural form”，the reinforcement methods were proposed.
