Abstract: It is the key of enlarging application of TEM to improve the interpretation accuracy.It has been put forward that the geoelectrical interface of underground medium can be recognized by reflection coefficient.The mathematics model has beem raised.Beginning with the data observed in field, calculating wave impedance with the technical of digital filter in frequency domain, and after domain transform functions that contain reflection coefficient have been constructed in time domain.At last reflection coefficient corresponding to each time gate has been computed.The section of the parameter has been drawn.The result of two numerical modeling and example application shows that the thought is right and efficient.The distinguishing capacity of TEM to geoelectrical layer is improved using the method.