Abstract: ZnSn(OH)₆ (ZHS) nanocubes were synthesized using a chemical precipitation method. Then a novel composite flame retardant, ZHS-MF was prepared by means of coating melamine-formaldehyde resin (MF) on the surface of the ZHS. Study on fire-retardant mechanism of ZHS-MF filled with flexible poly(vinyl chloride) (PVC). According to XRD, TEM, FTIR, TG results, MF resins have been successfully coated on ZHS surface. The mass fraction of ZHS is about 35%. The limiting oxygen index (LOI) results show that LOI of flexible PVC increases from 23.8% to 29.6% with the increasing of ZHS-MF, which indicates ZHS-MF is a highly effective flame retardant for flexible PVC. In the meantime, the MF shell could increase the flame retarding efficiency of ZHS-MF and decrease the dosage of nano-ZHS under the same LOI condition. The smoke density rating (SDR) results show that the SDR values of the samples treated with ZHS-MF are lower than that for those treated with ZHS alone when the addition of flame retardant ZHS-MF is 15%, which indicates the shell of MF and the core of ZHS have synergistic smoke suppression.