Abstract: Three-dimensional forward modeling in transient electromagnetic (TEM) surveys is a crucial tool for studying the response patterns of different scenarios and targets. A key issue in forward modeling is ensuring that the model closely approximates the actual geological conditions. This paper uses the EM3D software to model and conduct three-dimensional forward analysis of complex terrain and karst geological models, investigating the response characteristics of transient electromagnetic fields under complex geoelectric conditions. The three-dimensional forward simulation of complex terrain models reveals that complex terrain induces distortions in the transient electromagnetic response, and ignoring terrain effects can lead to false anomalies. The three-dimensional forward simulation of complex karst geological models shows that undulating strata and the boundaries of complex karst anomalies also cause distortions in the transient electromagnetic response, significantly interfering with anomaly identification. Analysis of the transient electromagnetic responses of the two complex geoelectric models indicates that the vertical induced electromotive force dB_z/dt effectively reflects low-resistivity bodies, the electric field E_y parallel to the source accurately delineates the boundaries of low-resistivity bodies, and the electric field E_x perpendicular to the source shows positive values within the anomaly and coexisting positive and negative values outside the anomaly.