The oil and gas industry has a significant demand for insulation and refrigeration in its pipelines. For example, the dehydration unit pipelines in natural gas processing plants and the high-temperature steam pipelines in refineries. In fact, insulated and refrigerated pipelines are more prone to corrosion failure than bare pipes. The purpose of this paper is to study methods for defect detection in coated pipes without damaging the aluminum coating, thereby overcoming the challenge of detecting coated pipes. By analyzing the force-magnetic coupling process of ferromagnetic materials, combined with the magnetization effect of ferromagnetic materials at high lift-off heights, we analyze the characteristics of defect magnetic signals at high lift-off heights. This clarifies the feasibility of detecting pipelines with coating layers. Research has demonstrated that within the elastic stress range, ferromagnetic materials can enhance the intensity of the leakage magnetic field when magnetized by an external magnetic field, which helps to overcome the impact of large lift-off heights on detection. By adjusting the structure of the magnetization unit, the magnetization effect on the pipeline can be further enhanced, thereby increasing the leakage magnetic field intensity. Ultimately, this can achieve the detection of pipeline defects at a lift-off height of 30mm or even higher, while avoiding damage to pipelines with coating layers. This paper innovatively achieves the detection of pipeline defects from outside the coating layer by improving the magnetization method. It overcomes the damage to the coating layer caused by common detection methods and avoids pipeline shutdown. Ultimately, it significantly reduces the fluid energy consumption during the detection process.