In this work, phosphorus-containing triazolyl amine (P-ATA) was incorporated into epoxy resin (EP) through the ring-opening reaction of epoxy groups, so as to obtain the intrinsically flame-retardant epoxy resin with excellent mechanical properties and high transparency. The modified EP with the addition of 7.5 wt % P-ATA obtained the limited oxygen index value of 33.1% with 32.5% increase compared to pure EP. Meanwhile, the self-intumescent flame-retardant action of EP/P-ATA was also evidenced by the remarkably reduced heat release and the expanded char of high yield, and the relevant flame-retardant mechanism of P-ATA was analyzed systematically. Moreover, EP containing 5.0 wt % P-ATA exhibited excellent mechanical properties with the enhancement of 16% of flexural strength, 17% of tensile strength, and 73% of charpy impact strength. Besides, EP-2.5% and EP-5.0% P-ATA maintained the transmittance more than 90% of neat EP, reserving its potential application in optical fields.