Abstract: In transient electromagnetic exploration,the depth of investigation is related to the observation time.The size of transmitter loop is mainly for strength of signal to satisfy reviver sensitivity and signal to noise ratio.When supplying a small transmitter loop device with power current,the strength of signal as same as that produced by a large transmitter loop can be obtained.At the pre-sent time,detecting precision is subject to the edge effect of large loop configuration and the small loop device avoids this objection.The practice prospecting results in Yunnan,Hunan and Xingtai show that the most deep depth of investigation is up to 800 m by small loop device and the interpretations are agreeable to the facts revealed in engineering,drill data and geological data very well.