Abstract: A mathematical analysis method of magnetic flux leakage(MFL) testing in pipelines cracks was proposed. In this method, firstly, the mathematical analysis model for MLF was derived based on the Maxwells equations. Then, smulation model was established under Ansys environment based on the 3D finite element(FE) theory. Furthermore, numerical simulation of MFL was made and agreed well with the actual experimental data. Finally, the effect of cracks geometric parameter, such as depth, width and so on. On the MFL signal characteristics were obtained by simulation analyzing, and their curves were given too. So this method is valuable to offer a reference in practical crack's MFL testing of pipeline.