As a device form of transient electromagnetic method, the ground-borehole TEM is being studied by more and more experts both in China and abroad. It has such advantages as small electromagnetic interference,strong signal for its receiving probe in the drilling or downhole tunnel, and closeness to the target anomaly. In this paper, the author used loop source as the excitation source, and established the geological model using finite-difference time-domain method (FDTD) with plate-like body and rectangular low-resistance body. The author calculated the transient response of the rectangular loop source ground-borehole TEM in the half space, and studied its characteristics and laws from the perspective of ground-borehole observation and ground-tunnel observation. The results show that, when multiple anomalies exist simultaneously, the ground-borehole TEM can distinguish different anomalies from the anomalous body and can accurately locate the depth and the horizontal position of different anomalies. The results can provide reference for the quantitative explanation of the depth and location of the anomaly with ground-borehole TEM.