This paper deals with the numerical simulation of the changes of apparent resistivity caused by topographic undulation based on the model of 2 D layered medium by using the boundary element method, and the general calculating method of apparent resistivity under the condition of abnormal surface topography in the horizontal layered medium and the influence of topography on geoelectric precursor are also discussed. The results show that the bigger the changes in topography, the less the information in apparent resistivity changes from deep depth. Thus, the proportion of precursory information could be reduced. From this point of view, big undulation in topography could be unfavorable for geoelectric resistivity monitoring.