The work aims to improve the safety of electrical packaging materials and prepare an epoxy composite with highly efficient thermal conductivity and flame retardancy. Melamine-formaldehyde resin prepolymer (MF) was employed to modify graphene (G)/blackphosphorene (BP) to synthesize nano-filler via in-situ polymerization, and then epoxy composites with high thermal conductivity/flame retardancy were prepared with the supplement of E51. The thermal stability, thermal conductivity and flame retardancy of the composites were tested by TGA, hot wire method and cone calorimetry, respectively. The research results showed that, when the mass fraction of MF@BP/G was 3%, the residual charring rate of epoxy composites was as high as 22.19%, which was an increase of 76.77% compared with pure epoxy composites; the thermal conductivity was increased to 0.257 W/(m∙K), and the increase rate was 27.86%; and the peak heat release rate, the total heat release, the peak smoke release rate and total smoke release decreased by 43.76%, 27.72%, 46.81% and 28.83%, respectively. MF@BP/G used as a functional filler can effectively improve the thermal conductivity and flame retardancy of epoxy composites, and is conducive to improving the use safety of epoxy composites.