Abstract: The transient electromagnetic method has a stronger adaptability to the surface of construction area,with a higher effectivity.Using the suitable apparatus the transient electromagnetic method could be used to prospect the underground cave.Its data processing and interpretation use the full time definition,transferring the measured transient attenuation curve to the apparent resistivity-depth curve,the existence of underground cave is determined based on the shape of iso-apparent resistivity curve analyzed by the characteristics of transient electromagnetic response of underground cave.The better geologic effectivities had been obtained by this method through the practical application in many times.In the old workings prospects in Mentougou area,Beijing,in 1993 and 1997,the old workings delimited by the transient electromagnetic method had the 80% of success ratio of borehore verification,proved that this method is available in prospect of underground cave.