Abstract: Submicrometer-sized cobalt hydroxystannate (CHS) flame retardant was prepared via coprecipitation method and applied to flexible polyvinyl chloride (PVC) to obtained CHS/PVC composites. The flame retardancy, thermal stability and mechanical properties of CHS/PVC composites were investigated by limiting oxygen index (LOI), cone calorimeter, TG analysis and universal tensile tests. The results show that the CHS effectively enhances the flame retardancy and maintains the mechanical property of CHS/PVC composites. Compared with blank PVC, when the mass ratio of CHS to PVC is 10%, the LOI of CHS/PVC composite is increased by 2.3%, the peaks of heat release rate and smoke production rate are reduced by 39.6% and 57.4%, respectively. The enhancement for CHS/PVC composites in flame retardation mainly attributes to the water dehydration from CHS when heating which dilutes heat. On the other hand, the CoCl₂ generated during the combustion process can effectively catalyze the early decomposition of PVC and then a denser and continuous residual charlayer is formed, thereby the combustion of PVC is effectively suppress.