Using XRD and TEM techniques, the modification mechanism of Fe3O4-SrFe12O19 particles has been studied. We have also studied the growth mechanism of spinel phase coating overlayer, and demonstrated that there exists a strong exchange coupling between coating overleyer and core particles due to the complexation of Fe2+ ions in suspension and Fe3+ ions extracted from the core particles. After modification, crystal structure and magnetic properties of SiFe12O19 particles were modified.