Halogen-free flame retardance high impact polystyrene (HIPS) was prepared by melt blending HIPS, nano modified aluminum trihydrate (nano-CG-ATH), encapsulated red phosphorus(ERP), and modified polyphenylene oxide (MPPO). The effects of nano-CG-ATH, ERP, and MPPO on flame retardance, mechanical, and thermal properties of HIPS composites were studied. The morphologies of char layers were characterized through SEM. The HIPS/nano-CG-ATH/RPM/MPPO (60/6/9/25) composite and its combustion residues at various temperatures were characterized by FT-IR spectra. The results show that nano-CG-ATH show a good synergistic effect of flame retardance with ERP and MPPO. The limiting oxygen index (LOI) and UL-94 rating of the HIPS composite reach 27.5% and V-0 respectively when the content of nano-CG-ATH, ERP, and MPPO is 6%, 9%, and 25%, respectively. The tensile strength and flexural modulus of the HIPS composite reach respectively 30.04 MPa and 2485.60 MPa. The residue for the HIPS composite reaches 10.58%. Its impact property needs to be improved.