Using geophysical methods, specifically transient electromagnetic (TEM), for CO2 monitoring is an effective way to detect CO2 diffusion. In this work, a multi-scale finite-difference time-domain (FDTD) algorithm was established to monitor CO2 by defining new iterative relations and approximating boundary conditions, which achieves unification in the time and space domain. The response curve characteristics of different forms of CO2 were acquired by changing the receiver's depth and position, CO2 resistivity, scale, and injection stage. Different models considering a planar, tilted, and large-scale CO2 bodies, which were established to test the capacity of TEM monitoring for CO2. The TEM response of injected CO2 bodies had obvious characteristics and the response curve had distinguishable differences from background. This phenomenon could provide reference models for real TEM CO2 monitoring.