To expand the application of ammonium polyphosphate (APP) in epoxy resin (EP), hyperbranched polyethyleneimines (PEI) were selected to modify it via cation exchange reaction. Then, a highly-efficient flame-retardant hardener with poly-functionalities for EP was successfully prepared and named as PEI-APP. After curing, PEI-APP endowed the EP samples with good flame retardance and smoke suppression performance. Results suggested the total heat release (THR) and total smoke production (TSP) decreased 76.1% and 70.5% respectively. Thermogravimetric analyses (TGA) of the PEI-APP cured EPs displayed a slight improvement in the high temperature region compared with the reference sample (PEI/EP). Dynamic mechanical analysis (DMA) demonstrated that the glass transition temperature (Tg) of PEI-APP/EP also slightly increased compared with PEI/EP. Fourier transform infrared spectra (FTIR) was used to analyse the condensed products of PEI-APP/EP samples at different temperatures to investigate the flame-retardant mechanism. All the aforementioned results distinctly confirmed that PEI-APP did not only act as an effective flame-retardant hardener for EP, but also brought a good thermal stability and improved the smoke suppression to the system. This polyamine hardener provided a new platform for intumescent flame-retardant application in EP.