Abstract: Based on the principle of magnetic flux leakage detection, the magnetic flux leakage signal characteristics of small-diameter thin-walled pipes with defects was simulated by using the finite element software COMSOL, and the change of magnetic flux leakage signals was studied when there were defects on both the inner and outer sides of the pipe wall. At the same time, the magnetic flux leakage field signal distribution of internal and external defects in the pipe wall was studied by using local DC magnetization and method of controlling variables. The results showed that, as the wall thickness increased to 6 mm, the overlap of internal and external defects with an overlap degree of 40% was no longer easy to determine; when the width of the defects was less than 2 mm, the differential change of double peak values was small in the tangential component distribution of the leakage field, which made it impossible to determine the parameters of the defects on the inner side. This result provided a basis for the study of the relationship between the overlap of the internal and external defects and the magnetic flux leakage signal, as well as the safety assessment of thin-walled pipes.