Ferromagnetism substances in the pipe pile will be magnetized under the excitation of geomagnetic field, which changes the magnetic field around the pipe pile. Ferromagnetism substances can be regarded as a right cylinder, and magnetic field characteristics around the pipe pile were analyzed on combination with model piles' tests. Lengths of pipe pile under buildings were detected by the magnetic logging principle.Application examples indicate that it is effective to detect the length of the pipe pile under buildings with the magnetic logging principle.