Abstract: In order to prospect gob water precisely,the authors studied the whole space transient electromagnetic response of gob water by numerical modeling using t hree dimensional finite difference time domain method. The numerical modeling results show that in the situation of normal direction of the transmitting loop being parallel with tunnel,the response curve amplitude of time derivative for gob water located in ahead of heading face is stronger than that located in the two sides of heading face. M oreover,curve minimum value of situation with gob water located in ahead of heading face is smaller than that gob water located in two sides of heading face as well as the response time of situation with gob water located in ahead of heading face is earlier than that gob water located in two sides of heading face. However,the shape of two cur ves are similar. In addition,for the gob water with different volumes and different distances from the heading face,the response curves are different. Finally,the effectivenes s of prospecting gob water of mining TEM is testified by drilling data,this shows that the mining TEM is one of very effective way to prospect gob water. .