Calculation of all-time apparent resistivity for the central loop TEM magnetic source devices is discussed in this paper. Homogeneous half-space calculation results show that it is feasible for transient electric dipole superposition to calculate the magnetic field source device TEM central loop transient field; the definition of all-time apparent resistivity for magnetic source central loop device using electric dipole transient field. Through the calculation of apparent resistivity and apparent resistivity for the two-layers, threeand four-layers electric section and comparison with late-time apparent resistivity for TEM, the superiority of the definition is illustrated. The calculations of magnetic source TEM field data show that the definition of alltime apparent resistivity for magnetic source central loop device using electric dipole transient field better reflects the characteristics of Electric section and it is valuable for shallow magnetic source TEM detection and data interpretation