The ground-tunnel transient electromagnetic method (TEM) is a geophysical prospecting method in which electromagnetic waves are emitted into the ground and measurements are taken in an underground tunnel. Based on the monotonic relationship between vertical magnetic induction intensity and apparent resistivity, this paper puts forward an idea for finding a unique solution and/or multiple solutions of the full-domain apparent resistivity of ground-tunnel TEM according to the inverse function theorem. It mainly involves calculating and analyzing the response characteristics of the full-domain apparent resistivity curves of four different geoelectric models H, K, A, and Q in different positions to changes in such parameters as formation resistivity. The results show that full-domain apparent resistivity calculated using vertical magnetic induction intensity can accurately reflect the electrical distribution laws of different geoelectric models. When the formation parameters and receiving point positions change, the full-domain apparent resistivity curve shape changes accordingly. The change is specifically expressed in the time order and amplitude, and can accurately reflect formation information. Finally, combined with the characteristics of ground-tunnel TEM, full-domain apparent resistivity with different offsets is calculated. The results show that full-domain apparent resistivity curves with different offsets are consistent, with little differentiation. The analysis of the full-domain apparent resistivity of ground-tunnel TEM can lay a theoretical foundation for the interpretation of this method.