One-pack flame-retardant epoxy resins (EP) with long pot life before using, high curing activity during operating and satisfactory fire safety after curing are extremely needed in electrical & electronics and composite industries. Here, imidazolium diphenylphosphinate (IDPP) was designed and synthesized by a facile way, thereafter used as a hardener for diglycidyl ether of bisphenol A type EP (DGEBA). IDPP/DGEBA possessed long-term stability in liquid state at ambient temperature while cured rapidly under heating, presenting controllable latent curing behavior relying on temperature. Specific phosphorus-containing groups were finally attached to epoxy chains mainly by ionic bonds, thus slightly affected the network structures of the resins. Consequently, the cured EP simultaneously achieved high glass transition temperature (Tg), good thermal stability, excellent flame retardancy and expected dielectric performance. Especially, with only 15 wt% IDPP incorporated, the cured DGEBA exhibited a high Tg (172.7 °C), low dielectric constant (4.00) and loss (0.004) at 10 MHz respectively, and easily achieved UL-94 V-0 rating with a high limiting oxygen index (37.0%). The data obtained from cone calorimetry indicated the flammability of the resins was significantly suppressed when cured with IDPP. It was verified by thermogravimetric analysis/infrared spectrometry that IDPP showed dominating vapor-phase flame-retardant effect in DGEBA systems. Additionally, IDPP was also qualified as a latent hardener for tetraglycidyl diaminodiphenylmethane (TGDDM) with excellent thermal stability and flame retardancy.