Semi-airborne transient electromagnetic method is a novel exploration method which is beneficial for deep sounding under complex terrain and geological conditions. Complex terrain will lead to electromagnetic response distortion. At present, there is still insufficient theoretical basis for topographic effect recognition in grounded-source semi-airborne transient electromagnetic data. Therefore, this paper studies the topographic effect of grounded-source semi-airborne transient electromagnetic method based on finite element method. In this paper, different three-dimensional terrain models are constructed, multi-azimuth flight lines are designed, and the abnormal response of magnetic field and induced electromotive force is quantitatively evaluated and analyzed. Three-dimensional terrain can cause anomalies in the observed responses over a wide area, with the anomalous regions not being limited to the vicinity of the actual terrain locations. The influence of terrain on dB_z/dt response is more complex than it on hz response. There are also notable distinctions in the characteristics of how terrain at the source location and terrain away from the source location influence the observed responses, with the former having a more pronounced effect on the observed responses. Additionally, the anomalies caused by ridge and valley in the observed responses are opposite.