Abstract: The transient electromagnetic method has played an important role in detecting the water bearing ahead of tunnels. Because of the existence of turn-off time in practical application, the exciting field can not satisfy the step turn-off condition. An expression for the step at the central point of small multi-turn loop is given based on the transient field of level electric dipole. Through the analysis of the transient field induced by three non-step turn-off currents, the influence range of the turn-off time is determined. It indicates that the detecting influence of the turn-off time on superficial part containing water ahead of the tunnel cannot be neglected. To improve the means for interpretation of water detection, the algorithm for the whole period apparent resistivity is derived in full space. Physical model tests and practical applications show that this improvement algorithm is more satisfactory to divide the space of superficial part containing water and increases the judgement capability of its aquosity.