The magnetellurics(MT) is a natural source electromagnetic technique. It has many advantages, such as more deep exploration, higher resolving power and survey efficiency. In exploring karst, studying geological structure and looking for hidden faults, especially in exploring tunnel of earlier stage, the MT is an effective method. Compared with the other geophysical meth ods, the exploration depth and resolving power of this method have a greater imp rovement. An application of the MT in the tunnel exploration of Hu-Rong nationa l road is presented. It is verified that this method can ascertain the geologica l status of some tunnel and has received good effect.