Based on the excavation of Chaodongyan tunnel， a numerical analysis concerning the influence of karst caves in the top of tunnel on stability of the surrounding rock masses is carried out by using the ANSYS software system; the yield initiation is assumed to occur following Druck-Prager failure criterion. The results of numerical modeling show that the stability of the surrounding rockmass of the top of tunnel increases with increace of the distance between the karst cave and tunnel; and reduces with increase of the karst cave size. There is a good relationship between the deformation of the surrounding rockmass and the distance from the karst cave to tunnel. The limited distance between the karst cave and tunnel is about 2—3 times of the diameter of karst cave. The character of stress field and plastic district of the tunnel surrounding rock masses is discussed.