To address the poor flame retardancy and low thermal conductivity of carbon nitride, in this paper, urea, p-formaldehyde, and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were utilized as raw materials to prepare DOPO-functionalized carbon nitride (CNSC-DOPO). It was then blended with ammonium polyphosphate (APP) to obtain a composite flame retardancy particle (APP/CNSC-DOPO), followed by fabricating epoxy resin composites (EP/CNSC-DOPO and EP/APP/CNSC-DOPO). The thermal conductivity, flame retardancy, and smoke suppression properties of these composites were investigated. The results reveal that when the CNSC-DOPO is added at an addition of 3wt%, the thermal conductivity of EP/3CNSC-DOPO reaches 1.013 W·m-1·K-1, representing a 264% increase compared to pure epoxy resin. Furthermore, When the mass ratio of APP to CNSC-DOPO is 12.5:1.5 and the total addition is 14wt%, the EP composite achieve a V-0 rating with an LOI of 32.4%. Compared to pure EP, the heat release rate (HRR), total heat release (THR), smoke production rate (SPR), total smoke production (TSP), and CO2 production (CO2P) are decreased by 74.39%, 44.16%, 67.65%, 52.13%, and 54.83%, respectively. The incorporation of CNSC-DOPO into the EP/APP formulations exhibits a synergistic flame-retardant effect, thereby effectively suppressing the release of heat and toxic fumes and significantly improving the flame-retardant and smoke-suppressing properties of the material.