Abstract: To solve the problem of the excessive dynamic range for receiving signals of helicopters by time domain electromagnetic method, concentric bucking coil was used to weaken the primary magnetic field in this paper. A model of magnetic field's distribution of concentric bucking system was established. Combining with the analysis of magnetic torque, the magnetic flux was analyzed by means of integral equation and nonlinear regression. It is concluded that the detection ability is weakened as the increase of the bucking coil. Through the analysis of the longitudinal magnetic field below the plane of the transmitting coil, it shows that the dynamic range of the receiving signal is reduced and the near-zone effect is weakened with the increase of the bucking coil. Therefore, the size of the bucking coil can be chosen basied on the receiving coil, accuracy of data acquisition system, as well as the flight altitude of the helicopter.