Abstract: In order to improve the heat resistance, flame retardancy and mechanical properties of traditional water-based polyurethane (WPU), in this study 9,10-dihydro-9-oxo-10-phosphenanthrene-10 oxide (DOPO) and 2,2’-diallylbisphenol A (DABA) were used as starting materials, and the chemical synthesized double DOPO pendant flame retardant DDBA, the chemical structure was characterized by infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR), and a new waterborne polyurethane (DDBA/WPU) was prepared by chemically modifying waterborne polyurethane (WPU). The effects of DDBA on water resistance, heat resistance, flame retardancy and mechanical properties of DDBA/WPU film were studied. Evaluation of the films by water contact angle, water absorption, thermal weight loss (TG), cone calorimetry (CONE), scanning electron microscope (SEM), oxygen index (LOI), vertical combustion (UL-94), universal testing machine related performance. The research results show that with the increase of DDBA addition, the water resistance, heat resistance, flame retardancy and mechanical properties of the film continue to improve. When the addition mass ratio of DDBA is 20%, the water contact angle can reach 134.56°, an increase of 106.06%, and the water absorption rate is reduced by 33.29%, the temperature resistance is increased by 60°C. Its LOI value is 35.9%, the total flue gas release (TSP) and average effective heat of combustion (AEHC) decreased by 85.42% and 55.76%. The maximum heat release rate (pHRR), total heat release (THR) and CO₂ release decreased by 35.40%, 51.20%, 58.49%, ignition time is prolonged by 15 s, CO release increased by 163.46%. The tensile strength can reach 25.7 MPa, which is about 9.51 times than that of WPU before modification.