Abstract: This paper analyses the influence of terrain of observation point on data collection and the conversed apparent resistivity of transient electromagnetic method(TEM), the amplitude of influence is not only associated with the change of the terrain, but also with surface resistivity and observation time delay. The simulation results accord with the experimental results, verifying the influence law of topography on the TEM data. The essence of the terrain effect on TEM data is that undulating terrain prompts eddy current to aggregate or diverge, resulting in the induction field to increase or decrease, making TEM apparent resistivity change with topography reversely. According to the law of the impact of topography on TEM data, the appropriate correction may be used.