The frequent drill fracture accidents during the drilling process severely affect the exploration and development of gas resources. To minimize this problem, this paper applies metal magnetic memory (MMM) detection technology to the fatigue damage detection of drilling tools. In this paper, 35CrMo steel, representative drilling tool material, was selected to carry out four-point bending fatigue tests on a self-designed fatigue testing machine. The surface morphology of specimens was observed under an optical microscope with a CCD camera and the crack propagation length was measured. At the same time, the changes in MMM signals in the fatigue process were collected by a high sensitivity MMM sensor based on the giant magnetoresistance principle. The characteristic parameters of MMM signals were extracted and the damage assessment was studied based on the magnetic memory characteristic parameters. The experimental results showed that the change of MMM signal parameters in the fatigue damage process of 35CrMo could be divided into four phases; initial stage, steady stage, fluctuation stage and rapid growth stage, during which the fluctuation stage corresponded to microcrack coalescence and expansion. MMM signal characteristic parameters can quantitatively assess the severity of fatigue damage of 35CrMo steel. This paper provides theoretical and data support for the MMM detection technique to be applied to the fatigue damage detection of drilling tools.