The full magnetization reversal process of perpendicular orientated Nd2Fe14B/α-Fe trilayer has been investigated by using a micromagnetic method. Both nucleation and pinning fields as well as hysteresis loops have been calculated reliably as functions of Ｌs （soft layer thickness）. In addition, detailed comparison between our results and experimental data has been done. With the demagnetization energy considered, the nucleation field of perpendicular by orientated case is lower than the parallel one and the nucleation occus before the reversal of the applied field. For small Ｌs, the theoretical coercivity equals to the nucleation field （it equals to pinning filed also）. As the thickness of the soft phase increases, the coercivity mechanism changes from nucleation to pinning. For large Ｌs, however, the coercivity is slightly smaller than the pinning field.