Abstract: Through the calculation of the primary field of transient electromagnetic method with the large fixed loop, the distribution and delay features of the primary field of different offset and different loop size were analyzed. The results of the research demonstrate that the early primary field was greater than that in the center within certain depth range, the more the offset within certain range is, the more the primary field and the more depth rate from ground to this depth are. Over this depth, the primary field of the center is the largest, when the offset is increased to some extent, the delay features of the different offset are different; rim effect has great influence on signals of shallow layer and the big length-width ratio rectangular loop. Finally, the study in the area where the target has the same depth and the error of primary field is no more than 5% solved the problem of how to delineate effective exploration area.