The flame retardant effect of single phosphaphenanthrene and triazine in epoxy resin (EP) is limited. In order to improve the flame retardant properties of EP, under the condition that the total mass fraction of flame retardant was 10%, 2,4,6-tris-(4-hydroxyphenylimino)triazine (MSB) and 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-sulfide (DOPS) were compounded at different ratios, the melt blending method was applied to prepare EP composites and the flame retardant properties was measured to investigate the synergistic flame retardant law. The results demonstrate that the limiting oxygen index (LOI) is 27.7% and 30.6% when MSB or DOPS were added separately, and the vertical combustion (UL-94) grade is no rating and V-1, respectively. When the total mass fraction of flame retardant is 10% and the mass fraction of DOPS in the composite flame retardant MSB/DOPS is 60%, the LOI value of the composite EP/MSB/DOPS is 29.7%, reaching UL-94 V-0 level. Compared with pure EP, the peak heat release rate (PHRR) and total heat release (THR) are reduced to 378.56 kW/m2 and 82.74 MJ/m2, respectively, dense and expanded char layer is formed after combustion, and the char residue content rising to 22.94%. The flame retardant performance of EP/MSB/DOPS-60wt% is superior to EP/DOPS or EP/MSB. It is shown that there is a good synergic flame retardant effect between phosphaphenanthrene and triazine flame retardant, which effectively improves the flame retardant performance of EP.