To obtain highly fire-safe epoxy resin (EP), piperazine-modified ammonium polyphosphate (PAz-APP) with multiple active –NH– groups was prepared and utilized as a highly effective flame-retardant hardener. After curing by PAz-APP as a monocomponent hardener, cross-linked networks containing both tertiary amino and ether linkages were obtained, which resulted in two glass transitions. Thanks to the phosphorus-containing inorganic part, PAz-APP brought excellent flame retardance and smoke suppression efficiency to the EP system. The cured sample passed V-0 rating (UL-94) with only 7.5 wt% addition of PAz-APP. Cone calorimetric results suggested that, compared with PAz/EP (as a reference sample), both the peak-heat release rate (PHRR) and total smoke production (TSP) of PAz-APP 15/EP (15 wt% addition) sharply dropped by 81.5% and 80.0%, respectively. By analyzing the chemical constitution of the decomposing residues at different temperatures, it was noticed that PAz-APP mainly acted as a flame retardant in the condensed phase via the formation of phosphorus-rich char. Dynamic mechanical analysis (DMA) illustrated that the main glass transition temperature ( T g ) of PAz-APP 15/EP was as high as 162.4 °C. Furthermore, the incorporation of PAz-APP did not worsen the mechanical properties, but contrarily, improved the impact strength.