Abstract: Based on the relevant parameters of the Q235A steel pipe for urban underground water pipeline, a 2-D model is established with the ANSYS software.The magnetic flux density curve as describing the magnetic flux leakage field in a defect is obtained by means of changing the inner surface defect length, defect depth and lift-off value of the model.And the distribution regularity of the leakage magnetic field of the inner surface of the pipe under different conditions is also obtained by means regularity of comparing the magnetic flux leakage curves.It is regarded that the flux leakage detection method as used for oil and gas pipelines can be also used for defect detection of metallic urban water pipeline.