The four stages of the demagnetization process of oriented sintered Nd-Fe-B alloy after saturation magnetizing along easy direction is studied theoretically in this paper. The results indicate that the nucleation in the soft magnetization regions located at the surface of hard magnetic grains, as well as the irreversible domain wall displacement from the surface to the inside of grains are the dominant factors controlling the intrinsic coercivity of Nd-Fe-B alloy. The rapid reduction of coercivity of Nd-Fe-B alloy with temperature rising are mainly attributed to the thermal movement of atoms it has a harmful effect on the perfection of tetragonal structure in hard magnetic grains and increases the thickness of the soft magnetization transition regions.