The aim of this study is to investigate the critical factor affecting the properties of PLGA     microspheres fabricated by a solid-in-oil-in-water (S/O/W) emulsion technique with BSA as a model protein.  Prior to encapsulation, the BSA microparticles were fabricated by a modified freezing-induced phase separation method.  The microparticles were subsequently encapsulated into PLGA microspheres by S/O/W emulsion method, then Motic BA200 biological microscope, confocal laser scanning microscope, scanning electron    microscope were used to observe the structure of S/O/W emulsion and PLGA microspheres.  The protein   content extracted or released from BSA microspheres was measured by Bradford protein assay method.  It   was found that NaCl added in the outer aqueous phase effectively suppressed material exchange between the  inner and outer phase of S/O/W emulsion.  Then, the structure and permeability of obtained microspheres were     influenced.  As a result, with the increase of NaCl concentration in the outer aqueous phase, the encapsulation efficiency of microspheres significantly increased from 60% to more than 85%, the burst release of microspheres reduced from 70% to 20%, and the particle size decreased from 103 μm to 62 μm.  Furthermore, the rehydration of encapsulated protein was also retarded and then integrity of BSA was successfully protected during encapsulation process.  In vitro release test showed that BSA released from PLGA microspheres in a sustained manner for more than 30 days.