Abstract: Large size carboxyl crosslinked polystyrene microspheres were prepared by activation swelling polymerization with polystyrene microspheres as seeds obtained by dispersion polymerization,1,2-dichloroethane as the swelling agent,dioctyl adipate（DOA） as the assistant swelling agent,divinyl benzene（DVB） as the crosslinking agent and methyl methacrylate（MAA） as the water-soluble functional monomer.Effect of the swelling agent type and size of active seeds on carboxyl crosslinked polystyrene microspheres’ largest average particle size and size distribution was researched and carboxyl crosslinked polystyrene microspheres were analysed by IR spectrum and thermogravimetic analysis（TGA）.Carboxyl crosslinked polystyrene microspheres and the largest size of 25.0um were obtained,and the thermolysis temperatures was 273.8℃.