The chopped glass fibers (GF) were first modified by 3-aminopropyltriethoxysilane (KH550) (GF-KH550), and then were coated by a layer of hyperbranched polyphosphamide (HBPN) to form interfacial flame-retarded GF@HBPN by in situ polymerization among phosphorus oxychloride (POCl3) and 4, 4-diaminodiphenylmethane (DDM) with triethylamine as acid binding agent and catalyst. Then a series of polylactic acid (PLA) composites including GF-KH550/PLA、GF@HBPN/PLA and APP/GF@HBPN/PLA composites were prepared by mixing various GF (GF-KH550 or GF@HBPN) with ammonium polyphosphate (APP). Their thermal, mechanical, and flame-retarding properties were systematically characterized and analyzed. Compared with neat PLA, GF-KH550/PLA composites exhibited significantly improved tensile strength but seriously worsened flammability caused by the obvious “candle wicking” effect of smooth GF, while GF@HBPN/PLA showed moderately enhanced tensile strength and improved flame-retardancy (passing UL-94 V-2 rating) due to the interfacial char residue formed by the pyrolysis of HBPN on the GF surface. To achieve a satisfied flame retardancy, APP was added into GF@HBPN/PLA composites. It is found that APP/GF@HBPN/PLA composite containing 10%APP and 30%GF@HBPN reached a high LOI value of 26.8% and achieved a UL-94 V-0 rating, moreover, its peak heat release rete (PHRR), total heat release (THR) and maximum mass loss rate (MLRmax) also decreased greatly in comparison with neat PLA, which were 31.39%, 23.57% and 49.26% respectively, illustrating superior fire safety of the above PLA composite.