FLAC3D is applied to analyze the stability and the ultimate bearing capacity of upper plate of karst cave under the loading of rectangular spread foundation; and two typical types of rock mass are studied. The ultimate bearing capacity of upper plate of karst cave in various engineering conditions are presented by analyzing the characteristics of rock stress and displacement and the whole evolvement process of plastic zones in the upper plate. The results show that the ultimate bearing capacity increases with the increasing of the thickness of upper plate and decreases with the increasing of the width of karst cave. In the same conditions
the ultimate bearing capacity of rock IV is only one third of rock III.