To obtain a latent curing epoxy system with satisfactory thermal stability, flame retardance and dielectric properties, imidazolium dibenzo[c,e] [1,2] oxaphosphate (IDOP) was synthesized by a facile way and utilized as a latent flame-retardant curing agent for epoxy resins (EP). It was confirmed that IDOP/EP one-pack system kept reactive inert near room temperature and cured efficiently under heating with a moderate heat release. The curing procedure was explored by X-ray photoelectron spectroscopy (XPS), confirming that the flame-retardant group was incorporated into epoxy chains by covalent and/or ionic bonds, hence the intrinsic flame retardance and excellent thermal stability were given to the cured resins finally. With only 15 wt% IDOP additions, the limiting oxygen index (LOI) increased to 37.0% from 20.5% of the reference sample, and UL-94 V-0 rating was achieved. The results of cone calorimetry test further certified that IDOP/EP showed satisfactory flame retardance dominating in gaseous phase, which was confirmed by the results of thermogravimetric analysis/infrared spectrometry (TG-IR). The thermal mechanical behavior of IDOP/EP was also evaluated by dynamic mechanical analysis (DMA). Especially, the incorporation of the flame-retardant group didn't deteriorate the dielectric properties of the cured resin. Benefiting from the advances, the latent curing epoxy system with excellent comprehensive performance exhibited potential for versatile applications.