In this work, polystyrene (PS)/triphenyl phosphate (TPP) nanocomposites were prepared by suspension polymerization. The TPP dissolved firstly in styrene monomer, and then suspension polymerization was carried out. The results showed that TPP nanoparticles, being about 50 nm spheres, were homogeneously and uniformly dispersed in a PS matrix, which was first reported. Furthermore, the formation mechanism of TPP nanoparticles was simulated by the dissipative particle dynamics (DPD) method. The flame retardance of the PS/TPP nanocomposite was significantly improved, which was attributed to the nanodispersion of TPP particles in the PS matrix.