A novel method has been developed to protect protein drugs in poly (lactic-co-glycolic acid) (PLGA) microspheres using S/O/O/W multi-emulsion method. This method develops a novel protein drug sustained-release system, which is based on the combination of protein-loaded dextran glassy microparticles (protein-loaded AqueSpheres) and PLGA microspheres. The protein molecules are encapsulated in the dextran glassy particles and the drug-containing dextran glassy particles are further dispersed in the PLGA microspheres. The protein-loaded AqueSpheres based PLGA composite microspheres have spherical shape and a smooth surface. They possess a normal size distribution and a mean diameter of 67.08 microm. The method may decrease protein aggregations and incomplete release due to avoiding protein contacting with oil/water interfaces and hydrophobic PLGA. The dextran glassy particles can stabilize proteins in the PLGA matrix, which is the major advantage of this novel protein sustained-release system over preparation for the PLGA microspheres using W/O/W double-emulsion method.