Traditional cured epoxy resin (EP) usually loses its transparency once it encounters the requirement of flame retardance. To obtain the EP with simultaneous excellent transparency and flame retardancy, a novel multifunctional polymeric curing agent named DPPEI was synthesized via a reaction between diphenylphosphinic chloride (DPPC) and polyethylenimine (PEI) in this work. Different measurements confirmed that the DPPEI was prepared successfully. After incorporation of the DPPEI into EP, the cured EP (DPPEI-EP) material with simultaneous excellent transmittance and flame retardancy was obtained after a mild curing process. The transmittance of the resulting DPPEI-EP was kept at about 90% in the visible region at a loading of 35 wt% DPPEI, meanwhile, the DPPEI-EP sample with the thickness of only 1.6 mm passed the V-0 rating in vertical burning test, had no dripping, and achieved the limiting oxygen index of 29.8%. In combustion test, both total heat release and total smoke production of the DPPEI-EP containing 30 wt% DPPEI were respectively greatly decreased by 69.5% and 78.3% compared with the corresponding value of the reference sample PEI-EP, showing high flame-retarding and smoke-suppressing efficiency. The transparent and flame-retardant mechanisms of DPPEI-EP were investigated insightfully through different tests. All these results demonstrate that the DPPEI is a novel multi-functional polymeric curing agent for EP, which has efficient curing ability and meanwhile endows the cured EP with simultaneous excellent flame retardancy and transparency.