Abstract: In recent years, the ground transient electromagnetic method has achieved good results in the field of mineral resources, hydrogeological exploration and other fields, and the reliability of the detection results decreases obviously when the surface condition is not good. One of the influencing factors is that the layout of rectangle loop with topographic relief makes its shape change. According to Biot-shaffar law, the magnetic field induced by electrified rectangular loop at any point in space is calculated. Through comparison and analysis of the distribution characteristics of three components of primary field of horizontal and undulating loop sources in subsurface media at different depth, it is concluded that the distortion of the primary field of the undulating loop source is obviously related to the shape of the source, its features are shown as serious distortion at shallow part, with increase of depth, the distortion weakens evidently. The above research provides theoretical guidance for understanding and correction of the topographic effects in transient electromagnetic exploration.