The present paper briefly describes the basic principle of electromagnetic shift and shift processing method of low-frequency filtering,and deals in detail with the technique for figuring out optimum extremum time T .With Straton-Chu intergral as the theoretical basis and by means of Green's function in infinite uniform media,the electromagnetic shift describes the propagation process of quasi stationary electromagnetic field in lower half space.Using the electromagnetic shift method to TEM data-processing,one can rather accurately locate local conductive geological bodies,benefit from high processing speed and obtain vivid interpretation result.