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A reactive flame retardant (DTP) is successfully synthesized and characterized, and then incorporated into epoxy resin (EP) as a co‐curing agent. The thermal stabilities and flame‐retardant properties of epoxy thermosets are evaluated. Compared to neat EP, the thermogravimetric analysis (TG) results show that the thermal decomposition rate of EP/DTP is reduced by 45% and the char yield at 750 °C is increased by 67%, when the addition amount of DTP is 12 wt%. In the cone calorimetry test (CCT), the values of the peak of heat release rate (pk‐HRR), total heat release (THR), total smoke produce (TSP), and average of effective heat combustion (av‐EHC) of EP‐DTP‐12 are exhibited a reduction of 36.6%, 26.4%, 40.5%, and 23.6% relative to neat EP, respectively. Meanwhile, the value of the limiting oxygen index (LOI) of EP/DTP‐12 can reach 34.83%, and 6 wt% or more loading of DTP make EP/DTP pass the V‐0 rating in the UL‐94 test. Moreover, the flame‐retardant mechanism is investigated by thermogravimetric analysis/infrared spectrometry (TG‐IR) and scanning electron microscopy (SEM), which can be summarized into two aspects: the quenching effect of phosphorus‐containing radicals and diluting effect of nonflammable gases in the gas phase, and the barrier effect of char layer in the condensed phase. [ABSTRACT FROM AUTHOR]