A rigid poly(vinyl chloride) foam with a cross-linked network structure was prepared by adding 3-glycidoxypropyltriethoxysilane (KH-561) into the universal formulation. The resulting foam had excellent heat resistance because of the increasing degree of cross-linking and number of Si–O bonds with a high heat resistance. The as-prepared foam was verified using Fourier-transform infrared spectroscopy (FTIR), energy-dispersive spectrometry (EDS) and foam residue (gel) analysis, which demonstrated that KH-561 was successfully grafted and cross-linked on the PVC chains. Finally, the effects of different KH-561 and NaHSO3additions on the mechanical properties and heat resistance of the foams were studied. The results showed that the mechanical properties of the rigid cross-linked PVC foam were raised after adding a certain amount of KH-561 and NaHSO3. The residue (gel), decomposition temperature, and chemical stability of the foam significantly improved compared to the universal rigid cross-linked PVC foam (Tg= 72.2 °C). The Tgof the foam could reach 78.1 °C without any mechanical degradation. The results have important engineering application value regarding the preparation of lightweight, high-strength, heat-resistant, and rigid cross-linked PVC foam materials.