Using cyanuric acid and calcium hydroxide/zinc oxide as raw materials and deionized water as solvent, zinc/calcium cyanurate, a new kind of poly(vinyl chloride) (PVC) thermal stabilizer, has been synthesized. Calcium/zinc cyanurate was characterized by elemental analyzer (EA), fourier transform infrared spectrometer (FTIR), thermogravimetry (TG) and scanning electron microscopy (FESEM). The thermal stability of calcium/zinc cyanurate as a thermal stabilizer for PVC was studied by oven aging test and Congo red test, and the mechanism of thermal stability was explored. The results showed that the addition of 1% of calcium cyanurate and zinc cyanurate to PVC alone, compared to the same amount of calcium stearate and zinc stearate, the former had a better thermal stability than the latter. The calcium/zinc cyanurate compound showed a good synergisitic effect, and the best quality ratio was 3:1. The PVC test piece was fully roasted with zinc for 70 min, and the thermal stabilization time reached 28 min. It has also been found that epoxy soya bean oil (ESBO) is an auxiliary stabilizer that is well suited for the calcium/zinc cyanurate system and can better improve the thermal stability of PVC systems.