In this study, the effects of sulfonated poly (styrene-divinylbenzene) (SP(St-DVB)) microspheres on the thermal curing of bisphenol A-based benzoxazine (Ba) was studied by preparing resin systems consisting of Ba and SP(St-DVB) microspheres. The thermal curing behavior and mechanism of Ba/SP(St-DVB) systems were investigated by differential scanning calorimetry and in situ Fourier transform infrared spectroscopy. The curing kinetics of Ba and Ba/SP(St-DVB) systems were investigated by non-isothermal DSC. The apparent activation energy of Ba and Ba/SP(St-DVB) systems gradually increased with the reaction extent (alpha). The dynamic mechanical analysis indicated that the glass transition temperatures of PBa/SP(St-DVB) composites did not decrease. The improved thermal stability of PBa/SP(St-DVB) composites was investigated by thermogravimetric analysis in the air and nitrogen atmosphere, respectively. The images of the fracture surfaces for PBa and PBa/SP(St-DVB) composites were investigated by scanning electron microscopy, demonstrating improved toughness of PBa/SP(St-DVB) composites.