Abstract: In this paper,a numerical method is used to simulate the evolution process of a karst groundwater system,and the phenomenon of the disappearing of a watershed divide during the evolution is discussed.The assumptions for this simulation is that the rock fractures are regarded as the smooth plane and the flow is laminar,so that we can apply the cubic law to describe the flow in the rock fracture network.An empirical equation is used to represent the erosion effect,and the flow is assumed to be under steady state at every time step.The results show that the erosion effect for each fracture is not unique,but different. The most developed part in the whole karst system is located near the water table,and the development of the vertical fracture is more obvious than the parallel one.The karst in the deep part develops very slowly,and the increasing rate for the fracture width in the most developed part is more than 8200 times bigger than that in the deep part of the system.The difference in the erosion effect causes the prior flow conduit.Until the disappearance of the watershed,more than 90% drainage is from this single prior flow conduit.The time needed for the watershed disappearance from the karst system is affected by the scale of the karst zone and the rainfall.When the scale of the karst zone increases 5 times the original one,the required disappearance time increases 10 times accordingly. This means the larger the scale of the karst zone is,the harder the watershed disappears.As for the rainfall influence,with the increasing in the rainfall,the required disappearance time increases firstly and then decreases.