A series of flame-retardant waterborne polyurethanes were synthesized with 4,4'-diphenylmethane diisocyanate and polyethylene glycol600 (PEG600) and modified with different level of diethylbis(2-hydroxyethyl) aminomethylphosphonate (fyrol-6); besides, 1,1,1-Tris(hydroxymethyl) propane was introduced as chain extender and crosslinking agent. The structures of these waterborne polyurethanes were confirmed by Fourier transform-infrared spectroscopy. Thermal degradation properties and flame retardancy were investigated by thermal gravimetric analysis (TGA), limiting oxygen index (LOI), and vertical burning test (UL-94). Wettability and mechanical property of film were measured with contact angle analyzer and universal mechanical machine, respectively. The results showed that when the mass of fyrol-6 was 12.0 % and the TMP was 1.7 %, an LOI value of 28.1 % and a UL-94V-2 rating could be achieved. Besides, the mechanical properties, including tensile strength, 100 % modulus and impact strength, are also relatively good. Simultaneously, the TGA analysis indicated that the incorporation of fyrol-6 enhanced the carbon residue from 8.9 to 19.9 %.