In order to study the stability of deep-buried cavity in Karst Area，the collapse mechanism of deep-buried rectangular cavity beneath a karst cave was established based on the upper bound theorem for limit analysis. The generalized Hoek Brown failure criterion was introduced into the virtual power equation of upper bound calculation. Analytical equation of collapse curve of surrounding rock at the top of cavity and limit excavation width were obtained by variational method，and the influence of parameters was analyzed. Taking Wuzhishan tunnel of Lebai expressway as the engineering background，the collapse range of surrounding rock at the top of tunnel was numerically simulated， and the results were compared with the upper bound solution calculated in this study. The results show that the collapse range of the surrounding rock near the cave top and the limit excavation width are affected by the parameters of surrounding rock，the limit excavation width increases with the rising GIS index. The numerical simulation results agree with the upper bound solutions..