Abstract: This article discusses the fundamental FEA theory of electricity magnetic field. The magnetic flux leakage field in MFL detecting in oil and gas transport pipeline is described, and a special example is analyzed with ANSYS software.The space distribution, the strength of magnetic indu ction, the signal chart of magnetic flux leakage field are gained. The work mentioned above is helpful to understand the magnetic flux leakage field in the space. The foundation of quantitative detection of def ects in oil and gas transport pipeline is established.