Accurate localization of subsea pipelines is the prerequisite of real-time tracking and elaborate inspection by underwater robots. Magnetic anomaly generated by the ferromagnetic pipeline can be used to locate both uncover and buried pipelines. However, due to low signal ratio and model inconstancy under weak and variable ambient magnetization, there is still missing an intuitive and reliable pipeline localization method in order to realize pipeline tracking. This paper proposes a method capable of immediately and accurately locating pipelines via active magnetization and vertical magnetic measurement. Finite element simulation shows that magnet array can significantly enhance the magnetic anomaly, and the vertical magnetic component alone can accurately indicate pipeline position, avoiding the inconvenience of magnetic three-component alignment in field. It is experimentally demonstrated that magnetic detection SNR can be significantly increased by 5dB~20dB for a Φ219mm steel pipe by using the magnet array, and the max lateral positioning error is 0.03m and much smaller than that without magnet.