In order to improve the flame retardancy of epoxy resin (EP), the flame retardants with double functional groups, hexa (4-(9,10-dihydro-9-oxa-10-hosphaphenanthrene-10-sulfide)-hydroxymethylphenoxy)cyclotriphosphazene(HAP-DOPS) and hexaphenoxy cyclotriphosphazene (HPCTP)/9,10-dihydro-9-oxa-10-phosphophene-10-sulfide(DOPS) with 1.8 wt% phosphorus content were respectively added to EP by melt blending. The effects of intermolecular and intramolecular synergistic flame retardancy of phosphazene and phosphoheterophene on EP were studied, and the flame retardancy, thermal degradation behavior and char residues of HAP-DOPS/EP with different mass fractions of HAP-DOPS were investigated. At last, the flame retardant mechanism was analyzed. The experimental results show that both HAP-DOPS and HPCTP/DOPS can effectively improve the flame retardancy of EP. But HAP-DOPS is better in reducing heat release, and the potential fire risk of HPCTP/DOPS/EP is higher than that of HAP-DOPS/EP. The results of char residue analysis show that HAP-DOPS/EP has better char forming ability and char layer quality than HPCTP/DOPS/EP, and exhibits excellent flame retardant effect in condensed phase. The study of flame retardant mechanism shows that when HAP-DOPS is added to EP, its diluting effect in gaseous phase, quenching effect of free radical and shielding effect of char layer in condensed phase play a synergistic flame retardant effect. Therefore, the flame retardant HAP-DOPS plays a synergistic flame retardant role both in the gas phase and condensed phase, while HPCTP/DOPS mainly in the condensed phase.