For raising the testing height as far as possible to detect micro weld seam by using magneto-optical（MO） sensor,firstly,the leakage magnetic field modeling analysis of micro weld seam is studied when the weld seam is magnetized by a DC magnetic field,secondly weld seam MO image can be obtained by putting a MO sensor above close surface of the weld seam,thirdly,through comparative studying between magnetic field intensity distribution feature of weld seam surface and the transitional zone feature of micro weld seam MO image,a micro weld seam MO imaging rule and a raising height way which can test micro weld seam as far as possible by using MO sensor are confirmed. Micro weld seam leakage magnetic field distribution model setting up by magnetic charge theory can effectively explain the weld seam MO image transitional zone information feature and it can provide a theory basis way to seek the weld seam from weld seam MO image for laser welding seam tracking and identifying and to find a way for raising the testing height by using MO sensor as far as possible.