Abstract: When the probe is on the edge of the specimen, there are some deviations between the ideal model and the specimen, which result in the measurement error. Based on the PET detecting principle, effects of the specimen thickness, lift-off height on the measurements are numerically simulated when the probe is on the edge of ferromagnetic specimens, and are compared with experimental results. The results show that the numerical simulations are well consistent with that obtained in the experiments. The measurements values are always smaller than the actual thicknesses when the probe is on the edges of the specimens. The measurement errors increase with the increases of the thickness and the lift-off height. The relative error obtained using the probe in the specimen corner area is about twice that using the probe in the general edge away from the corner area. In order to reduce the measurement error caused by the edge effect, fitting formulas are finally given, by which the actual specimen thickness can be obtained using the measured thickness and lift-off height.