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In view of the detection requirements of pipeline internal and external surface defects and pipeline special regular structure detection, based on the magnetic shielding effect inside and outside the pipeline, the principle of magnetic field refraction at the interface between the pipe wall and air, and the principle of the magnetic field concentration on the pipe surface and the gap, this article focuses on the natural environment (geomagnetic background). The magnetic field distribution inside and outside the pipeline has been theoretically analyzed. After that, the finite element simulation software is used to model the finite length pipeline, and the internal and external magnetic fields are simulated and researched in combination with the finite element method. Finally, the tunnel magneto resistance (TMR) magnetic sensor is used to collect and process the internal magnetic field of the actual pipeline model, which verifies the correctness of the theoretical analysis. This paper analyzes the magnetic field inside and outside the pipeline by means of principle deduction, finite element simulation, and sensor measurement. The analysis results show that the distribution of the magnetic field in the pipeline under the excitation of the geomagnetic field has a very obvious regularity, which can meet the needs of pipeline defect detection. In addition, because the pipelines of different structures have different magnetic field distribution laws, they can also be used for the detection and analysis of pipelines with special structures.