The author queries in this paper the theory that the TEMS exploration depth depends on the strength of the loop's magnetic moment (M=IL²), and that under the circumstance of limited field source power, the depth will be effectively controlled by loop area. This paper holds that the opinions of "big loop area makes large depth" and "small loop area makes shallow layer " are not correct in that they are the results of applying the depth formula without considering the premise condition. The author thinks that the TEMS exploration depth is decided by frequency parameter of the field source, that the lower the frequency, the larger the exploration depth, and that enough magnetic moment is required to produce high signal-to noise ratio in the measurement.