Abstract: Located in the city of Xianyang in Shaanxi Province, the Hanyang Mausoleum is the first fully closed underground museum in China and in the world. The fissure disease of its outer pit is developing, which seriously endangers the safety of the pit wall and the pit bottom cultural relics, and it is urgent to protect and strengthen the pit. In the past, more was learned about the disease and prevention of the ruins in the open air and semi-closed environment, while the completely closed environment was seldom examined. This paper takes the outer pit K21 of the Hanyang Mausoleum as the research object. The number and distribution characteristics of cracks are determined through field investigation. On this basis, the causes of cracks are studied and classified. The stability of pit wall and its influencing factors are analyzed by numerical simulation methods. The research results show that there are 53 cracks in the K21 outer pit, mainly occurring on the south and north sides of the surface and the pit wall. The surface longitudinal cracks are unloading cracks, the surface transverse cracks are dry shrinkage cracks, and the pit wall cracks are mainly structural cracks. The longitudinal unloading crack is the main reason for the reduction of pit wall stability. The earthquake, artificial load or the further development of cracks will lead to the pit wall to be unstable. It is recommended to use anchor bolts to reinforce the pit wall, and reset and pre-tighten the unloading cracks by applying prestress. This study is of important guiding significance for the investigation and protection of the fissure disease in the museum sites.