The transient electromagnetic field of loop sources is solved by using the first kind of modified Bessel function integral, and the transient value in cylindrical coordinate system is obtained by using Faraday electromagnetic induction principle. The layered recursive formula of one-dimensional whole-space medium and the diffusion law of uniform whole-space transient electromagnetic field were studied, and the whole-space response characteristics of different loop sizes and classical models were analyzed and applied to the measured data. The results show that the transient electromagnetic field generated by the loop source has a certain directionality in space, and the increase of loop size does not affect the early response strength, but delays the corresponding delay time. The relative error of the whole-space layered solution is less than 4%, which has high accuracy, and when the loop radius is less than the layer thickness, the distortion of the low resistivity layer is basically at the same time, otherwise it increases. The reduction of radius significantly reduces the late response intensity, but expands the late application range and enhances the longitudinal resolution. The theoretical content of the coincident loop TEM is complemented, and reference for the working parameters of the whole space observation is put forward.