Abstract: As surveying with Schlumberger array, an approximate homogeneous electric field can be built in the middle one-third of the length of the electrodes AB stricken into the earth. This ground building pattern can be applied in the electric advanced forecast in tunnel excavation. In the middle of electrodes AB, those abnormal geological bodies with low resistivity or high induced polarization will result in the electric field or the induced field abnormity. The forecast method by measuring the abnormity is a new way to predict the hidden trouble ahead the tunnel excavating face and the precision is definite. The method overcomes the inconvenience as supplying electric power in the excavating tunnel and fits for the hidden trouble forecast in mines such as coal mine, the tunnel engineering of highways, water conservancy and hydropower.