Magnetic flux leakage detection is one of the most widely used non-destructive testing techniques in online pipeline inspection. The prerequisite for obtaining ideal results in this detection technique is that the excitation system magnetizes the pipeline to a saturated state. This requirement presents significant challenges in the detection of large-wall-thickness pipelines. On the one hand, a large excitation system is needed to achieve sufficient magnetization intensity. On the other hand, the oversized excitation system and the strong magnetic attraction it generates have become the major bottlenecks restricting the practical application of this technique in such scenarios. To address this problem, a defect detection technique based on the magnetic eddy current effect under low magnetization intensity is proposed. This technique focuses on detecting magnetic permeability anomalies at defect locations. The variations in magnetic permeability at defect sites in ferromagnetic materials under low DC magnetic fields are investigated, and the mechanism analysis and the mechanism of inner and outer wall defect detection based on the magnetic eddy current effect is analyzed through finite element simulation. A magnetic eddy current sensor was designed, and comparative experiments on magnetic eddy current and magnetic flux leakage detection of inner and outer wall defects were conducted on 15mm thick steel plates with artificially machined defects of different types and depths (50%, 40%, 30%, 20% and 10% of wall thickness). Experimental results show that when the magnetization current is between 0.7 A and 1.1A, the magnetic eddy current detection technique not only effectively identifies defects of varying depths and types and distinguishes between inner and outer wall defect locations, but also achieves significantly better detection performance than magnetic flux leakage detection under the same conditions. Both simulation and experimental results demonstrate that inner and outer wall defects in ferromagnetic materials can be detected under low magnetization intensity using the magnetic eddy current effect, verifying the feasibility and effectiveness of the proposed detection technique and providing an effective method for the full-wall thickness defect detection in large-wall-thickness pipelines.