TiO2/PO4 3- solid superacid was synthesized by using the precipitation immersion method and characterized by means of X-ray diffraction (XRD) and an energy-dispersive spectrometer; it was added into flexible poly(vinyl chloride) (PVC) composites as a flame retardant. The smoke suppression and flame retardance of TiO2/PO4 3- accompanied by Sb2O3 were investigated through the limiting oxygen index (LOI), cone calorimetry test, and thermogravimetry analysis (TGA). The results indicated that the LOI value of PVC/Sb2O3/(TiO2/PO4 3-) reached to 32.3%, which is higher by 6% than that of neat PVC (26.3%). In addition, Sb2O3/(TiO2/PO4 3-) greatly reduced the peak heat release rate and total heat release simultaneously in comparison to using Sb2O3 separately, suggesting a significant synergistic effect between TiO2/PO4 3- and Sb2O3 on improving the flame retardancy of the PVC. Further, results of TGA and differential thermal analysis showed that the thermal stability of the composites was greatly improved. For the Fourier transform infrared analysis, Sb2O3/(TiO2/PO4 3-) leads to a large amount of functional group surplus, which specifically indicates that its carbon residue increases. The surface of the char formed after combusting of the PVC compounds was observed through scanning electron microscopy. It is found that the solid superacid can promote decomposition, pyrolysis, and cross-linking of PVC into the compact and continuous char layer on the surface of the material, which improved the flame retardancy and smoke suppression of PVC.