Abstract: In this paper, TMR magnetic field sensor was used to detect corrosion under insulation in ferromagnetic pipes with pulsed eddy current testing. To improve detection accuracy, effect of geomagnetic field on testing was studied. A pipe with 70mm insulation was placed to the east-west and experiments were performed with TMR magnetic field sensor placed at different positions, over the pipe, the side of 45 degree from the top and the side of 90 degrees from the top respectively. The experimental results show that probe orientation has a significant influence on the detection . In actual testing, if receiving direction of TMR with respect to the geomagnetic field is changed greatly, reference standard should be reselected.