In order to realize the reliable inspection of metal loss defects in buried steel pipelines by using the external inspection method in China, the non-excavation inspection method for metal loss defects in steel pipelines based on multi-frequency electromagnetic is studied. By means of theoretical analysis coupled with finite element simulation, the distribution characteristics of alternating current at different frequencies flowing through the inner and outer wall defects of the pipeline are studied. The changes of the magnetic field around the pipeline caused by the inner and outer wall defects are then analyzed. Finally, an experiment is conducted to verify the feasibility of this inspection technology. The results show that: 1)When the alternating current with multiple frequencies is applied at both ends of the buried steel pipeline, if there is a metal loss defect in the buried pipeline, an abnormal signal occurs in the flow of alternating current. There is a significant difference between the frequency of occurrence of abnormal alternating current flow due to inner and outer wall defects. Similarly, the degree of abnormality of different frequency current flow caused by different depth of metal loss defect is also different. 2)When the alternating current begins to change as it flows through the defective pipe section, the alternating magnetic field of the corresponding frequency around the pipeline will also change. The identification and quantification of metal loss defects can be achieved by inspecting and analyzing the variations in the signals of various frequency electromagnetic field around the pipeline. 3)The experimental results are consistent with the finite element simulation results, therefore demonstrating that the non-excavation inspection technology based on multi-frequency electromagnetic can be used for the inspection of metal loss defects in buried steel pipelines. This research provides a basis and reference for the development of non-excavation detection equipment for metal loss defects of buried steel pipelines based on multi-frequency electromagnetic technology.