In the case of a certain geoelectric section, we analyze the diffusion depth of TEM field, and simulate the time when the peak of electric field excited by different coils emitting step pulses reaches a certain depth underground, all of which indicate that the arrival depth of TEM diffusion field is only related to the delay time. This lays a theoretical foundation for the TEM method of small center loop to detect greater depth. If the equivalent sending current of transmitter and the equivalent area of receiving coil or probe are increased, the TEM method of small loop device can reach the detection depth equivalent to that of large loop device in a certain range. This paper tries to demonstrate the detection ability of small center loop method from the combination of theory and practice.