Abstract: According to the operational characteristics of the ground pipeline and based on the theory of magnetic flux leakage testing technique, an automatic detection device on the outside of pipeline was designed, which was suitable for different diameters of pipelines. Then the corrosion defects with different depth and diameter on the wall of pipeline were simulated and tested through the finite element simulation and experimental research, and thereafter the influence law of different parameters on the leakage magnetic field was obtained. The results of simulation and experiment are consistent. In addition, according to the results of defect detection, the designed magnetic flux leakage detector is effective for the detection of defects both in inner and in outer walls of the pipeline.