A novel phosphaphenanthrene oxide, based on bridged 9,10-dihrdro-9-oxa-10-phosphaphenamhrene-10-oxide (DOPO) derivatives (PN-DOPO), was synthesized and incorporated into glass-fiber-reinforced polyamide 6T (GFPA6T) composites. Moreover, a commercial product of aluminum diethyl-phosphinate (OP1230) was introduced as a comparison to examine the effects on the flame-retardant properties and thermal stabilities of GFPA6T. UL-94 test and cone calorimetry results displayed that PN-DOPO showed better flame retardancy of GFPA6T than OP1230. Scanning electronic microscopy results of the char residues after UL-94 test indicated that PN-DOPO promoted a compact char formation. More importantly, the activation energies and the kinetic parameters of the decomposition process were calculated through the Kissinger-Akahira-Sunose (K-A-S) and Flynn-Wall-Ozawa (F-W-O) methods, and the thermal degradation mechanism was studied with the Coats-Redfern method based on thermogravimetric analysis at multiple heating rates. The results demonstrated that the E values of GFPA6T/PN-DOPO at the early decomposition and the late decomposition are higher than those of GFPA6T and GFPA6T/OP1230, especially in the late decomposition stage. The fact indicates that the higher E value and more dense carbonization protection layer contribute to the superior flame retardancy of PN-DOPO for GFPA6T.