Two micromagnetic models, Model A with the polycrystalline microstructure and Model B with just regular mesh, are set up to simulate the hysteresis loops of Co/Pt multilayers with perpendicular anisotropy. In both models, a defect region on a scale of 100 nm is introduced in any Co film with only two atomic layers, which plays a key role in this simulation. The magnetic anisotropy constant K of cells in the defect region is weakened compare to the K out of the defect. The simulated M-H loops express the characteristics of the experiments well. The loops of a series of [Co/Pt]n multilayers with different repetition period also agree with experiment well. Moreover, the movement of reversed domain wall during magnetization reversal is observed in this simulation.