In this report, we studied the formation mechanism of cagelike polymer microspheres fabricated conveniently and efficiently through a swelling-osmosis process of sulfonated polystyrene (SPS) microspheres in a ternary mixed solvent (water/ethanol/heptane). The scanning electron microscopy and transmission electron microscopy observations indicated that the morphology of the final cagelike SPS microspheres is mainly controlled by the composition of the mixed solvent and the swelling temperature. Considering the solubility parameters of related reagents and the low interface tension of heptane and the aqueous solution of ethanol (only 6.9 mN/m), we confirm that the porogen procedure starts from the swelling of SPS microspheres by heptane, followed by the osmosis process of water molecules into the swollen SPS microspheres forced by the strong hydrophilicity of -SO3H group. The water molecules permeated into SPS microspheres will aggregate into water pools, which form the pores after the microspheres are dried. These prepared cagelike SPS microspheres are further served as the scaffold for the in situ generated CdS nanoparticles under γ-ray radiation. The CdS/SPS composite microspheres show good fluorescence performance. This work shows that the cagelike SPS microspheres have a wide industrial application prospect due to their economical and efficient preparation and loading nanoparticles.