The coercivity dependence of sintered Nd-Fe-B permanent magnet alloys an orienting magnetic field was studied. The results indicate that demagnetization process of the magnet is basically the nucleation process of anti-magnetization in the soft magnetization regions located at the surface of magnetic grains，and the process in which the nuclei grow into domains and irreversible domain wall displacement takes place from the surface to the inside of grains. We compared the value and the variation dependence of nucleation field with that of depinning field on the orientation of magnetic field. we come to the conclusion that depinning field is the main mech-anism controlling the coercivity of sintered Nd-Fe-B permanent magnet alloys.