Abstract: 3D FEM simulation on the magnetic flux leakage (MFL) signals of internal defects in oil/gas pipeline is made with ANSYS, the distributionlaw of MFL signals accompanied with different shape defects is analyzed and the relationship between MFL signals and the model sensor's lift-off value with different shape and size defects. Results show that the distribution of MFL of defect is not only related to the seriousness of the defects, but also to their 3D shapes. The defects with different shapes have different optimum lift-off values. After the lift-off value is reduced to a certain value, the smaller the lift-off value is, the bigger the signals detected is, and the errors induced by the fluctuation of liftoff value will be accordingly reduced.