The structural deformation of subsea pipelines increases the risk of pipeline fracture, which can cause significant economic losses. This paper proposes a pipeline displacement and deformation detection method based on a spherical inner detector equipped with a multi-channel magnetic sensor array. The stress in the pipe wall and the change of the magnetic flux density inside the pipeline caused by the displacement deformation of the pipeline are analyzed according to the magneto-mechanical effect. The structure layout of the detector and the magnetic signal acquisition scheme are designed. The collection of spatial rotational magnetic flux density information inside the pipeline is realized, and the rotational magnetic signal is converted into translational magnetic signal in the pipeline coordinate system. By extracting the magnetic data on the contour lines inside the pipeline, multiple channels of signals are integrated to increase the space sampling coverage of the magnetic signal inside the pipeline. Finally, internal inspection tests were conducted on pipeline vertical and equivalent lateral loading-induced displacement deformation. The results showed that under longitudinal force deformation state, the magnetic flux density longitudinal component dispersion decreased by 136.1 μT on the contour line inside the pipeline; under equivalent lateral force deformation state, the magnetic flux density lateral component dispersion decreased by 123.1 μT on the contour line inside the pipeline. The magnetic flux density dispersion measured by the spherical internal detector can reflect the displacement deformation of the pipeline in different directions, and the larger the deformation of the pipeline, the smaller the magnetic flux density dispersion.