Abstract: When ferromagnetic wear debris was in the coil of inductive wear debris sensor , the magnetic field distribution of the coil was changed through magnetization , and hence the equivalent inductance of the coil was changed The magnetic model of the coil containing ferromagnetic wear debris was built The expressions between demagnetization factor and induction density were deduced Taking spherical wear debris for examples , the analytic expression of the rate of change of inductance was obtained by computing the magnetization field of spherical wear debris The magnetic field of coil was also computed by finite element method After analyzing the model and computing the magnetic field , the theoretical analysis was consistent with the numerical computation It means that the model was reasonable The magnetization intensity of wear debris depended on magnetic permeability and demagnetization factor , and the magnetization intensity of spherical wear debris could be approximately regarded as a constant that had no reference to its magnetic permeability The rate of change of sensor coilinductance induced by ferromagnetic spherical wear debris decreased with increasing number of tums per unit length , and approached a limit The rate of change of inductance was proportional to the cube of the radius of the wear debris When the radius was less than 100μm , the rate of change of inductance was of 10-7 order The conclusions of this study provide theoretical direction for design of inductive wear debris sensor