An efficiently brominated flame-retardant system which is composed of decabromodiphenyl oxide (DB) and antimonous oxide (AO) was used for long glass fiber-reinforced polypropylene (LGFPP), and the thermal stability, flame retardancy, and mechanical properties of DB-AO/LGFPP composites were investigated. When 12 wt% of DB-AO flame retardant was added into LGFPP, the limiting oxygen index of composites could reach 24.8 and pass the V-0 rating in UL-94 test. The results showed that DB and AO improved the flame retardancy of LGFPP more efficiently which could be proved by thermogravimetric test that had less effect on mechanical properties than widely used intumescent flame retardant to flame-retardant LGFPP. What’s more, analysis of cone calorimeter tests data indicated that gas phase flame-retardant mechanism exists in the DB-AO/LGFPP composites.