Abstract: At present, the processing of constant source loop transient electromagnetic data is mainly to calculate the apparent resistivity by one-dimensional inversion. In order to realize fast 3D forward and inversion of transient electromagnetic data in time domain and improve inversion accuracy, the 3D forward and inversion of ideal model and measured data in frequency domain is studied based on time-frequency conversion theory, regularization technology, finite difference method and NLCG inversion algorithm. The problem of matrix singularity in the process of time-frequency conversion is solved. The transformation from time domain to frequency domain of transient electromagnetic response, as well as the three-dimensional forward and inversion of transient electromagnetic response frequency domain are realized successfully.