Abstract: The paper deals with some problems of detection of small loop transient field:under certain geoelectric section, theoretic diffusion depth of transient electromagnetic(TEM) field depends only on time; meanwhile the same emission current of different size of loops is simulated, the peak of the excited underground electric field arrives at certain depth at the same time, but with different intensity; deduction and calculation showed that the induced emf of the loop center was bigger than the preconceived one, the conversed apparent resistivity of small loop observations will be greatly reduced when calculated with the formula for big loop. The results of several actual detection with small loop TEM method were presented. The authors try to show detection capability of the small central loop method from the combination of theory and practice.