Abstract: Water-filled goaf of abandoned small coal mine has always threatened the safety of coal mine in normal pro-duction. In order to effectively identify the hidden water-filled goaf beside a borehole and obtain its spatial location,the downhole transient electromagnetic method (TME) that transmitting step magnetic field on the ground and receiving three-component induction field in the borehole was studied. Geoelectricity models of water-filled goaf were constructed and 3D forward modeling were carried out. The anomalous responses of water-filled goaf with different parameters were calculated. The characteristics of anomalous response and the influence of parameters,such as resistivity,scale and rel-ative borehole distance,were analyzed. The results show that all three components of anomalous field respond obvious-ly,and the zero-crossing points of X-and Y-components and the extreme points of Z-components correspond to the depth of the water-filled goaf. The amplitudes of X-and Y-components are relatively small,but they are pure anomalous fields,which are easy to be identified and extracted. The amplitude of Z-component is relatively strong,but the anoma-lous field is one thousandth of the background field,which is difficult to be identified and extracted. Due to the closer proximity to the anomalous body,the Z-component acquired by the downhole TEM is stronger than that collected by ground TEM. The amplitudes of three components anomalous response increase as the water-filled goaf resistivity de-creases,the scale increases,and the distance decreases. Based on the equivalent current filament theory,a least square constraint inversion algorithm was developed,which can obtain the key parameters such as central coordinates,azi-muth,inclination and size of the anomaly by fitting the anomalous field. An experiment of detecting water-filled goaf was carried out in a coal mine in Yulin city,Shaanxi province,China,using downhole TEM. The X-and Y-components of collected data show obvious anomalous response characteristics near the target layer. By inversion fitting the abnor-mal field data of 1. 0 ms and 1. 3 ms,it is found and inferred that there is an abandoned small coal mine water-filled goaf on the north side of the borehole. The detection results are verified by the later drilling,which is consistent with the actual disclosure. Through theoretical analysis and field test,it is proved that the downhole TEM can effectively de-tect the water-filled goaf beside the hole,realize “ one hole for multi-purpose”,overcome the “ one hole view”,and provide a new geophysical exploration method for the fine exploration of the water-filled goaf of abandoned small coal mine.