For the defects with same dimension but located at different places the MFL signal amplitude differs greatly. In order to get accurate evaluation of the defect dimension, the primary task is to discriminate the internal defects from external ones. For the conventional MFL apparatus, magnets distance is relatively large, so the magnetic field can penetrate to internal surface of the pipe and distributed uniformly in the pipe. When the two magnets are close enough, the magnetic field concentrates at the external surface, and is weakened at the internal surface. The reduction of magnetic flux density at internal surface causes the decrease of MFL signal, thus makes the internal defect unable to be detected. The combinational use of two inspection units, one with short magnets distance and the other with long magnets distance, can effectively discriminate the external and internal defects.