The solid/liquid composite flame retardant zinc borate-tris (2-chloroethyl) phosphate-magnesium hydroxide-aluminum hydroxide (ZB-TCEP-MH-ATH) were introduced into dicyclopentadiene (DCPD) solution to prepare the DCPD stable dispersion, and then the flame retardant polydicyclopentadiene (PDCPD) composites were prepared by frontal ring-opening metathesis polymerization (FROMP). The effects of the composition and content of the composite flame retardant on the polymerization kinetics of DCPD, thermal stability, mechanical and flame retardant properties of the PDCPD composites were studied. The results show that the composite flame retardant will affect the reaction activity of DCPD and reduce the polymerization enthalpy and curing degree of the materials. With the increase of the amount of flame retardant, the polymerization enthalpy decreases more obviously, however, for the composites with the same flame retardant system, the curing degree of the material is only affected by the type of flame retardant, and the storage modulus and glass transition temperature of the material are linearly related to the curing degree of the materials. Also, the introduction of composite flame retardants reduces the tensile strength of PDCPD composites, but significantly improves the thermal stability, elongation at break, impact strength and flame retardancy of the materials. When the content of ZB-TCEP-MH-ATH is 55%, the limiting oxygen index (LOI) of the material reaches 41% and passes UL-94 V-0 test. In particular, the peak heat release rate (pHRR) and the fire growth rate (FIGRA) values of the material are only 8.51% and 3% of PDCPD, respectively, and the peak smoke production rate (pSPR) is reduced by 80.36%, and the fire performance index (FPI) is increased by 8 times, which shows good comprehensive performance.