Abstract: In MFL inspection, as the thickness of the ferromagnetic material to be detected or the buried depth increases, flaws are more and more difficult to be found. In order to search the ultimate, the detectability was studied experimentally on a thick steel plate with blind holes in different depth. Signals under different magnetizations were given. High SNR MFL signal was realized in the test of  2 mm×20 mm blind holes whose depth was up to 70 mm on condition that the magnetization was approaching to saturation，which could provide guidance for thick wall steel tube .